UC-NRLF 


SB    IflS 


OF  THE 

UNIVERSITY 

OF 


ABORATORY     MANUAL 
F  ELEMENTARY  COLLOID 
CHEMISTRY 


EMIL  HATSCHEK 


With  20  Illustrations 


PHILADELPHIA 

P.  BLAKISTON'S  SON  &  CO. 

1012  WALNUT  STREET 
1920 


H33 


Printed  in  Great  Britain. 


PREFACE. 

ALTHOUGH  most  of  the  existing  text-books  of  Colloid 
Chemistry  necessarily  give,  in  more  or  less  detail,  descrip- 
tions of  experimental  procedure  and  instructions  for  making 
many  of  the  classical  preparations,  no  laboratory  manual 
or  collection  of  practical  exercises  such  as  has  been  found 
indispensable  in  the  teaching  of  other  branches  of  chemistry 
has  so  far  appeared.  The  lack  of  such  a  work  is  all  the  more 
likely  to  check  the  spread  of  a  practical  knowledge  of  the 
discipline,  as  many  of  the  methods  and  materials  of  colloid 
chemistry  are  peculiar,  and  strange  even  to  students  well 
trained  in  inorganic  and  organic  chemistry. 

The  present  work  is  an  attempt  to  fill  this  gap  and  to 
supply  accurate  and  very  detailed  directions  for  carrying 
out  the  fundamental  operations,  for  making  a  number  of 
representative  preparations,  and  for  examining  them  by  the 
standard  methods.  These  are  based  throughout  on  personal 
experience  of  the  processes  described  and  of  the  difficulties 
experienced  in  teaching  them.  The  examples  chosen  are, 
generally  speaking,  the  simplest  ones  and,  where  alterna- 
tives are  possible,  those  involving  the  smallest  expenditure 
in  apparatus  and  material.  The  task  of  selection  has  not 
been  easy,  and  the  attempt  to  delimit  the  elementary  region 
of  the  whole  domain  may  seem  premature  or  arbitrary  :  the 
guiding  principle  has  been  to  provide  for  the  wants  of  those 
students  of  numerous  branches  of  science  who  are  finding 
some  training  in  colloid  chemistry  an  indispensable  part  of 
their  equipment,  and  are  able  to  devote  a  limited  time  only 
to  acquiring  its  technique. 

For  the  guidance  of  readers  desirous  of  going  beyond  the 
limits  of  this  manual  a  number  of  references  to  recent 
literature  are  given  at  the  end  of  each  section.  The  papers 
quoted  are  mostly  records  of  experimental  investigations 
which  are  either  alternative  to,  or  more  advanced  than,  the 
examples  given  in  the  text. 

Since  the  book  is  the  first  of  its  kind,  the  author  will  be 
very  grateful  for  hints  from  readers  who  may  find  any  of 
the  directions  given  in  it  lacking  in  clearness  or  capable  of 
being  simplified. 

EMIL   HATSCHEK. 

LONDON, 
February,  1920. 

426255 


CONTENTS. 


PAGE 

PREFACE       .........        v 

CHAPTER  I. — GENERAL  REMARKS  ON  APPARATUS,  MATERIALS 

AND  PROCEDURE       ......         9 

Choice  of  vessels,  methods  of  cleaning  the  same. 
Making  up  solutions  and  sols.  Filtration  of  sols.  Dis- 
tilled water.  Redistilled  water.  Variability  of 
materials  and  degree  of  accuracy.  General  hints. 

CHAPTER  II. — DIALYSIS          ......       16 

Graham's  dialyser.  Parchment  bags.  Arrangements 
for  continuous  flow.  Parchment  tubes.  Parchment 
thimbles.  Collodion  thimbles.  Flat  collodion  mem- 
branes and  "  Star  dialyser."  Dialysers  for  sols  show- 
ing osmotic  pressure. 

CHAPTER  III. — SUSPENSOID  SOLS  .....  29 
Gold  sols  :  reduction  by  tannin  ;  reduction  by  for- 
maldehyde. Palladium  sol.  Silver  sol :  reduction  by 
dextrine  ;  reduction  by  tannin  ;  reduction  by  hydro- 
gen. Various  methods  of  making  gold  and  silver  sols. 
Sulphide  sols  :  cadmium  sulphide  sol ;  arsenic  sul- 
phide sol.  Miscellaneous  sols :  Prussian  blue  sol ; 
ferric  hydroxide  sol. 

CHAPTER  IV. — SUSPENSIONS  ......       37 

Mastic  suspension.  Other  resins,  dragon's  blood, 
gamboge. 

CHAPTER  V. — ORGANOSOLS     ......       39 

Reduction  of  silver  in  wool-fat.  Other  metals  in  the 
same. 

CHAPTER  VI. — EMULSOID  SOLS  AND  GELS  .  .  .41 
Silicic  acid  sol,  preparation  from  water-glass.  Salts 
promoting  setting  of  sol.  Determination  of  SiO2. 
Effect  of  lyotropic  salts.  Gelatin.  Commercial  raw 
material.  Swelling  and  dispersion.  Filtration  of  sols. 
Hardness ;  determination  of  melting  and  setting 
points.  Examination  of  strained  gels  in  polarized 
light.  Agar.  Swelling  and  dispersion.  Filtration. 
Agar  gel.  Effect  of  lyotropic  series  on  setting  and 
swelling.  Purified  gelatin.  Sols  containing  a  definite 
amount  per  volume. 

CHAPTER  VII. — EGG  ALBUMIN  SOL  ....  57 
Preparation  and  filtration  of  sol  from  dried  albumin. 
Heat  coagulation.  Coagulation  by  adsorption.  Salt- 
ing out  and  the  Hofmeister  series  of  anions.  Reversal 
in  acid  sols.  Heavy  metal  precipitation.  Purifica- 
tion of  sol  made  from  dried  albumin  and  from  fresh 
white  of  eggs.  Crystallized  albumin.  Dialysis  of 
albumin  sols  to  prevent  dilution. 


CONTENTS.  vii 

PAGE 

CHAPTER  VIII. — EMULSIONS  ......       63 

Pure  oil-water  emulsions.  Electrolyte  coagulation  and 
clearing.  Concentrated  emulsions.  Preparation  with 
alkali.  Separation  and  phase-ratio.  Use  of  soap  solu- 
tions and  simple  apparatus  for  same.  Phases  of  equal 
density. 

CHAPTER  IX. — ULTRA-FILTRATION  .....       69 
Apparatus   and   membranes   for   Bechhold's   method. 
Ostwaid's  ultra-filters  :    for  use  with  vacuum  ;    spon- 
taneous.    Thimbles  as  ultra-filters. 

CHAPTER  X. — OPTICAL  METHODS  OF  EXAMINATION  .       76 

Arrangements  for  observing  Tyndall  cone  and  state  of 
polarization.  Ultra-microscopic  and  dark-ground 
examination.  Jentzsch  ultra-condenser.  Dark-ground 
condensers.  Method  of  cleaning  slides  and  cover 
glasses 

CHAPTER  XI. — CATAPHORESIS  -  .          .         .         .81 

Simple  U-tube  apparatus.  Nernst  and  Coehn's  U-tube. 
Determination  of  velocity  in  unit  gradient.  Micro- 
scopic observation  and  measurement  of  cataphoresis. 
Method  of  preparing  slide.  Determination  of  velocity. 

CHAPTER  XII. — ELECTROLYTE  PRECIPITATION  OF  SUSPEN- 

SOID  SOLS         .......       89 

Typical  limit  concentrations.  Method  of  determining 
same.  Titration.  Precipitation  with  constant  sol- 
concentration.  Standard  electrolyte  solutions.  Pro- 
cedure. Standard  electrolyte  solutions  for  positive  sols. 

CHAPTER  XIII. — MUTUAL  PRECIPITATION  OF  SUSPENSOID 

Sois        ........       94 

Precipitation  in  definite  ratios.  Experimental  proce- 
dure. Determination  of  optimum  ratio  of  oppositely 
charged  sols.  Electric  charge  on  non-precipitated 
mixtures 

CHAPTER  XIV. — PROTECTION          .....       96 
Precipitation  in  presence  of  protecting  agent.     Gold 
numbers.    Experimental  procedure  for  determining  the 
same.    Specific  nature  of  protection. 

CHAPTER  XV. — VISCOSITY  MEASUREMENTS  .  .  .  100 
Types  of  capillary  viscometers.  Ostwald  viscometer. 
Determination  of  relative  viscosities.  Use  and  dimen- 
sions of  instrument.  Series  of  viscometers  with  increas- 
ing bore  of  capillary.  Cleaning.  Necessity  of  constant 
temperature.  Regulator  and  troubles  experienced  with 
toluene  type.  Elimination  of  density.  Ubbelohde  vis- 
cometer. Manostatic  arrangement  for  same.  Method 
of  using  apparatus.  Concentration-viscosity  and  tem- 
perature-viscosity curves. 

CHAPTER  XVI. — ADSORPTION  (QUALITATIVE  EXPERIMENTS)     no 
Adsorption  of  dyes.     Adsorption  of  lead  salts.     In- 
fluence   of   solvent.      Electric    adsorption.      Selective 
adsorption 


viii  CONTENTS. 

PAGB 

CHAPTER  XVII. — CAPILLARY  ANALYSIS  .         .         .         ,     113 
Description  of  method.    Detection  of  methyl  orange  in 
vegetable  dye  solution.    Detection  of  picric  acid. 

CHAPTER    XVIII. — DETERMINATION    OF   AN    ADSORPTION 

ISOTHERM         .         .         .         .         .         .         .116 

Choice  of  oxalic  acid.  Experimental  procedure.  Con- 
centration of  permanganate  solution.  Ti  trad  on. 
Example  of  actual  titration.  Plotting  y-C  and  log  y- 
log  C  diagrams.  Discussion  and  determination  of  ex- 
ponent. Comparison  of  different  solutes.  Difficulties 
of  analytical  methods.  Attainment  of  equilibrium. 
Choice  of  adsorbent. 

CHAPTER  XIX. — THE  LIESEGANG  PHENOMENON  .  .124 
Original  formula  for  silver  chromate  rings  in  gelatin. 
Calcium  phosphate  in  gelatin.  Lead  iodide  in  agar. 
Lead  chromate  in  agar.  Molar  concentration.  Indirect 
formation  of  rings.  Reactions  in  silicic  acid  gel.  Pre- 
servation of  specimens. 


NAME  INDEX 132 

SUBJECT-MATTER  INDEX 133 


A     LABORATORY    MANUAL 

OF    ELEMENTARY    COLLOID 

CHEMISTRY. 

CHAPTER  I. 

GENERAL  REMARKS  ON  APPARATUS, 
MATERIALS  AND  PROCEDURE. 

THE  apparatus  employed  in  the  operations  to  be 
described  is,  with  very  few  exceptions,  that  available 
in  any  chemical  laboratory.  Glass  vessels  used  for 
preparative  work  should,  if  possible,  be  of  resistance 
glass  ;  this  applies  even  to  test  tubes  used  for  such 
work  as  experiments  on  electrolyte  coagulation. 
Test  tubes  which  turn  distilled  water  containing  a 
little  phenolphthalein  pink  in  a  very  short  time  are 
by  no  means  uncommon,  and  should  not  be  used 
for  any  purpose.  As  regards  the  choice  of  larger 
vessels,  it  should  be  remembered  that  very  thorough 
cleaning  is  necessary,  and  that  in  many  cases  undue 
exposure  of  solutions  to  air  is  undesirable,  so  that  the 
choice  will  fall  on  tall  cylindrical  beakers,  conical 
beakers  with  spout,  or  Erlenmeyer  flasks.  Flasks 
with  narrow  necks  are,  generally  speaking,  undesirable. 

Vessels  should  be  cleaned  immediately  after  use, 
in  any  event,  and  again  before  use  in  the  case  of 
sensitive  preparations.  The  methods  to  be  adopted 
in  the  former  case  naturally  depend  to  a  great  degree 
on  the  previous  contents  of  the  vessel.  Suspensoid 
sols  are,  of  course,  easily  washed  off,  although  in 


io  GENERAL.  REMARKS. 


some  cases — especially  with  positive  sols — small 
quantities  are  adsorbed  on  the  glass  surface  so 
tenaciously  that  washing  with  dilute  hydrochloric  or 
nitric  acid  may  be  required  to  remove  the  adsorbed 
film.  Very  thorough  and  repeated  washing  is 
necessary  after  emulsoid  sols  ;  it  must  be  continued 
until  the  last  water  shows  no  trace  of  froth.  Traces 
of  gelatin,  albumin,  etc.,  allowed  to  dry  in  glass 
vessels  are  very  troublesome  to  remove,  and  may 
require  the  use  of  nitric  acid  or  hot  dichr ornate- 
sulphuric  acid  mixture.  Irreversible  gels,  e.g.,  silicic 
acid,  formaldehyde-gelatin,  etc.,  should  be  made 
only  in  vessels  from  which  the  gel  can  be  easily 
removed,  such  as  cylindrical  beakers,  preferably  with 
thick  walls.  Whatever  the  method  of  cleaning,  the 
vessels  should  finally  be  rinsed  thoroughly  with  dis- 
tilled water  and  drained.  Drying  with  cloths  is  to 
be  avoided  ;  drying  with  alcohol  and  ether  is  only 
necessary  in  the  case  of  small  apparatus  used  for 
quantitative  work,  e.g.,  viscometers.  Apparatus 
cleaned  as  described  and  kept  with  the  opening  down- 
wards will  generally  require  only  rinsing  with  several 
lots  of  distilled  water  before  use. 

Operations  like  making  up  salt  solutions  of  known 
concentration  for  the  preparation  of  sols,  electrolyte 
coagulation,  etc.,  require  the  usual  apparatus  and 
call  for  no  special  remarks.  Sols  of  emulsoids  con- 
taining a  definite  amount  of  dry  material  to  a  given 
volume  of  dispersion  medium  also  offer  no  difficulty. 
Substances  which  disperse  in  the  cold,  like  albumin 
or  gum  arabic,  stick  to  the  walls  of  the  vessel  in  the 
earlier  stages  of  the  process,  but  are  easily  detached 
when  swelling  has  progressed  sufficiently.  The  use 
of  thick-walled  vessels  is,  however,  advisable,  as 
thin  beakers  are  easily  broken  in  trying  to  detach 
fragments  which  stick  obstinately.  If  sols  contain- 
ing a  definite  weight  of  substance  in  a  given  volume 
of  sol  are  required,  the  procedure  is  a  little  more 


FILTRATION.  n 

difficult.  For  gelatin  it  is  fully  described  under  that 
heading  ;  with  materials  like  albumin,  gum,  etc.,  it 
will  be  found  advisable  not  to  make  the  sol  directly 
in  the  measuring  flask,  as  stirring  is  impossible.  The 
weighed  quantity  of  dry  material  should  be  placed 
in  a  beaker  and  dispersed  with,  say,  50  or  60  per  cent, 
of  the  total  volume  eventually  required,  and  the  sol 
so  obtained  poured  into  the  measuring  flask.  The 
beaker  is  then  carefully  rinsed  with  small  successive 
lots  of  dispersion  medium,  which  are  poured  into  the 
flask  :  the  aggregate  volume  of  these  washings  must 
fall  short  of  the  mark  by  a  few  cubic  centimetres. 
The  flask  is  finally  filled  to  the  mark  with  the  disper- 
sion medium  from  a  pipette  and  the  contents  well 
mixed.  This  method  is  not  quite  accurate,  but  the 
error  is  generally  not  as  great  as  that  due  to  the 
variable  moisture  content  of  the  starting  material. 

Filtration  will  be  necessary  chiefly  in  the  case  of 
organic  emulsoid  sols.  It  is  generally  a  somewhat 
tedious  process  and,  whenever  possible,  should  be 
left  overnight.  The  residues  which  have  to  be  re- 
moved are  generally  not  crystalline,  and  the  use  of 
vacuum  does  not  accelerate  the  rate  of  filtration 
materially  beyond  the  first  few  cubic  centimetres. 
Carefully  folded  filters  may  be  used  where  only  the 
filtrate  is  required,  as  the  complete  removal  of  residue 
from  such  filters  is  not  easy.  Ribbed  or  corrugated 
glass  funnels  utilize  the  paper  surface  better  than 
smooth  ones,  but  are  not  so  easily  cleaned.  All 
funnels  should  have  the  spouts  cut  off  quite  short, 
say  not  more  than  2  cm.  below  the  cone,  as  the 
usual  long  spouts  are  difficult  to  clean.  Fairly  hard 
filter  papers  are  advisable  in  most  cases. 

Small  quantities  of  troublesome  sols  may  be 
filtered  through  shredded  asbestos  with  good  results. 
This  can  be  used  in  the  ball  tubes  listed  in  most 
catalogues  of  chemical  glassware  in  the  following 
manner.  A  disc  of  silver  foil  is  cut  which  will  pass 


12 


FILTRATION. 


easily  through  the  upper  part  of  the  tube  (Fig.  i), 
and  this  is  perforated  with  a  strong  needle,  being 
supported  on  a  cork  plate  for  the  purpose.  The  disc 
is  then  placed  in  the  lower  part  of 
the  ball  and  pure  shredded  asbestos 
packed  into  it  up  to  its  junction  with 
the  tube.  The  asbestos  must  be  intro- 
duced in  small  quantities,  moistened 
with  the  solution  to  be  filtered,  and 
rammed  down  lightly  with  a  stirring 
rod ;  the  exact  degree  of  pressure 
required  can  be  found  only  by  experi- 
ence. The  tube  is  then  filled  with 
the  liquid  to  be  filtered  and  the  first 
few  cubic  centimetres  of  filtrate  re- 
turned to  it,  if  turbid.  Concentrated 
sols  like  those  of  albumin  or  gum 
arabic,  if  filtered  in  this  manner,  will, 
of  course,  generally  be  still  opales- 
cent, but  will  be  sufficiently  clear  in 
moderate  thicknesses  to  allow  the 
effect  of  coagulants  to  be  seen  dis- 
tinctly, and  will  be  free  from  particles 
which  would  interfere  with,  say,  vis- 
cosity measurements. 

A  centrifuge  capable  of  dealing 
with  at  least  100  c.c.  at  a  time  will 
be  found  a  very  useful  piece  of  appa- 
ratus, but  is  not  indispensable. 

A  microscope  provided  with  a  J"  and 
a  Y  objective  and  with  at  least  one 
high-power  eyepiece,  is  required  for  the 
examination  of  sols  by  dark  ground 
and  ultra-condensers.  The  special  features  of  these 
appliances  are  fully  dealt  with  in  the  section  devoted 
to  them,  but  a  general  knowledge  of  the  microscope 
must  be  presumed. 
As  regards  materials,  the  most  important  one  is 


FIG.  i, 


DISTILLED   WATER.  13 

pure  distilled  water.  If  a  sufficient  amount  of  con- 
ductivity water  is  available,  all  difficulties  are 
avoided.  It  is,  however,  not  essential  for  the  work 
described  in  the  following  pages,  and  the  extreme 
precautions  taken  in  the  case  of  a  few  preparations 
prominently  mentioned  in  the  literature  have  pro- 
duced a  somewhat  exaggerated  impression  of  the 
standard  of  purity  required  for  less  delicate  work. 
Water  distilled  with  ordinary  care  in  reasonably 
designed  apparatus  will  answer  for  all  but  a  few  pur- 
poses. Trouble  is  much  more  likely  to  be  caused 
through  the  ordinary  storage  vessels,  since  they  are 
very  rarely  made  of  resistance  glass.  If  trouble  is 
experienced,  the  first  thing  to  do  is  to  use  freshly 
distilled  water  only,  and  to  collect  the  small  quan- 
tities which  will  be  required  in  resistance  glass  flasks. 
Storage  vessels  coated  inside  with  paraffin  wax  may 
be  used ;  remember  that  the  lining  cannot  be 
removed  again  by  warming,  as  the  glass  cracks 
before  the  wax  melts. 

For  the  few  preparations  which  require  exception- 
ally pure  water  small  quantities  may  be  redistilled 
and  condensed  in  a  silver  cooler.  A  thin-walled 
silver  tube  about  f "  bore  is  not  expensive,  and  can 
easily  be  fitted  to  a  Liebig  condenser  in  place  of  the 
glass  cooler.  There  must,  of  course,  be  no  parts  on 
which  water  can  condense,  leading  down  the  cooler  : 
in  other  words,  the  cooler  must  be  bent  down  to  the 
distilling  flask.  This  may  be  done  by  filling  the  tube 
completely  with  fine,  dry  sand,  corking  both  ends 
and  bending  slowly  over  a  cylindrical  object  of  3"  or 
4"  radius.  The  water  should  be  redistilled  from,  and 
collected  in,  resistance  glass  flasks. 

It  is  hardly  necessary  to  add  that  throughout  this 
book  water  means  distilled  water  of  the  standard 
quality  available ;  where  redistilled  water  is  essen- 
tial, or  tap -water  permissible,  this  is  specially 
mentioned. 


14  MATERIALS  AND  ACCURACY. 

As  regards  the  other  materials,  the  ordinary 
chemicals  call  for  no  remarks.  The  solutions  made 
from  them  for  such  investigations  as  electrolyte 
coagulation  are  molar  and  not  normal,  and  this  should 
be  borne  in  mind.  If  there  is  any  doubt  about 
crystals  containing  the  full  amount  of  water  of 
crystallization,  or  if  the  salts  are  anhydrous  but 
hygroscopic,  e.g.,  A1C13  or  NH4CNS,  the  solutions 
must  be  standardized  by  the  usual  analytical 
methods. 

Materials  like  gelatin,  agar  or  dried  albumin  are 
not  definite  chemical  individuals,  differ  slightly  or 
even  considerably  when  obtained  from  different 
sources,  and  contain  an  amount  of  moisture  which 
varies  perceptibly.  The  best  that  can  be  hoped  for 
is  concordant  results  ;  the  first  essential  for  this  pur- 
pose is  to  use  the  same  material  throughout  a  given 
investigation  and,  therefore,  to  start  with  a  sufficient 
stock  to  allow  for  all  contingencies. 

These  considerations  have  an  obvious  bearing  on 
the  degree  of  accuracy  to  be  aimed  at  in  weighing  and 
measuring.  Centi-  or  milli-molar  solutions  of  elec- 
trolytes may  be  made  up  with  the  same  care  as 
solutions  for  volumetric  analysis,  although  the 
operations  in  which  they  are  eventually  used  have 
not  a  sharp  end -point.  On  the  other  hand,  it  is 
unnecessary  (and  will,  fortunately,  also  be  found 
impossible  with  many  atmospheric  conditions)  to 
weigh  10  gm.  of  gelatin  to  fractions  of  a  milligramme, 
since  the  moisture  content  may  easily  vary  by  0-5  per 
cent,  of  the  total  weight  in  a  very  short  time.  No 
general  rules  can  be  given  which  would  be  an  adequate 
substitute  for  the  exercise  of  common  sense  in  this 
respect. 

A  few  general  hints  on  procedure — many  of  which 
may  appear  superfluous  to  some  reader  or  other,  but 
have  not  been  found  so  by  the  author — may  conclude 
this  introduction. 


VARIOUS   HINTS.  15 

Read  the  whole  chapter  before  beginning  any  of 
the  work  described  in  it ;  although  the  operations 
are  generally  put  in  the  order  in  which  they  succeed 
one  another,  it  is  well  to  have  a  complete  idea  of  the 
work  before  starting. 

Many  preparations  change  with  age  ;  do  not  make 
more  than  you  require  for  immediate  use  or  than  will 
keep  safely. 

Label  all  preparations  immediately  in  terms 
which,  if  not  entirely  correct  technically,  will  remain 
intelligible  to  yourself.  This  is  particularly  impor- 
tant in  the  case  of  series,  like  solutions  of  different 
concentrations,  Liesegang  preparations,  etc. 

Adapt  your  methods  to  the  peculiarities  of  your 
material.  For  instance,  when  told  to  dilute  a  5  per 
cent,  collodion  with  an  equal  volume  of  acetic  acid, 
do  not  put  the  highly  viscous  sol  in  the  measuring 
'essel  first  and  pour  the  thin  solvent  on  it,  but  proceed 
in  the  reverse  order. 

When  an  experiment  fails,  repeat  it  with  the 
alteration  of  one  factor  at  a  time.  If,  e.g.,  a  gold  sol 
tu  ns  out  purple  instead  of  red,  try  first  a  fresh 
beaker,  than  a  fresh  carbonate  solution,  and  so  on. 


CHAPTER  II. 
DIALYSIS. 

THE  cheapest  and  most  convenient  membrane  for 
dialysing  any  but  small  quantities — say  50  to  100  c,c. 
— is  parchment  paper.  This  is  readily  obtainable  >r. 
sheets  or  cut  in  squares  of  various  sizes.  It  varies  a 
good  deal  in  permeability,  and  only  an  actual  trial 
can  decide  whether  a  particular  sample  is  satisfact*  >ry 
for  a  given  purpose.  As  the  paper  is  fairly  brittle  in 
the  dry  state  it  should  be  kept  flat  or  rolled,  but  never 
folded. 

The  classical  method  of  employing  the  parchment 
membrane  is  that  used  by  Thomas  Graham,  who^e 
dialyser  may  be  found  in  all  catalogues  of  chemkai 
apparatus.  It  consists  simply  of  a  glass  cylinder 
open  at  both  ends,  one  of  which  is  provided  with  a 
rim  or  groove,  over  which  the  membrane  is  tied.  -  A 
circular  piece  about  2"  larger  in  diameter  than  the 
cylinder  should  be  cut  and  thoroughly  soaked  in 
water,  placed  centrally  over  the  rim,  carefully  turned 
down  over  it  all  round,  and  then  tied  with  a  1i  ; 
string.  Unless  this  is  done  with  care,  leakage  i:s 
take  place  through  some  of  the  folds  formed  ;  belo; 
a — possibly  valuable — solution  is  placed  in  the 
apparatus  it  should,  therefore,  be  tested  by  filling 
it  with  water  and  ascertaining  that  it  does  not 
escape.  (The  same  precaution  applies  to  all  dialysers 
to  be  described  in  this  chapter  /)  The  dialyser  is  sus- 
pended or  supported  in  a  vessel  filled  with  water, 
which  is  changed  from  time  to  time,  or  renewed 
continuously. 


PARCHMENT  BAGS.  17 

Parchment  bags  are  preferable  for  larger  quanti- 
ties, as  a  larger  surface  is  obtained  in  the  same  space. 
They  may  be  made  as  follows :  Cut  a  regular  hexagon 
and  soak  it  thoroughly  in  water.  Then  place  it 
centrally  on  the  bottom  of  an  inverted  beaker  or  jar, 
the  diameter  of  which  is  about  one-third  of  that  of 
the  inscribed  circle  of  the  hexagon.  Gently  pinch 
radial  folds  from  the  circumference  of  the  beaker  to 
the  corners  of  the  hexagon  and  mould  them  so  that 
the  paper  midway  between  the  corners  touches  the 
wall  of  the  beaker,  and  then  turn  the  folded  portions 
over  and  smooth  them  into  cylindrical  shape.  The 
whole  procedure  will  be  quite  clear  from  Fig.  2, 
which  shows  the  initial  hexagon  (dotted)  and  the 
final  outline  of  the  edge  in  plan,  as  well  as  a  per- 
spective view  of  the  nearly  completed  bag  on  the 
beaker.  The  folds  must  not  be  sharp,  as  even  wet 
parchment  may  be  damaged  by  too  drastic  treat- 
ment. When  the  bag  has  been  moulded  as  described, 
a  string  is  loosely  tied  round  it,  or  a  fairly  slack 
rubber  band  slipped  over  it  within  about  2"  of  the 
edge,  and  the  bag  is  then  drawn  off  the  beaker.  Its 
permanent  shape  is  secured  by  threading  a  clean, 
thin  string  through  the  folds,  as  indicated  by  dotted 
line  in  the  plan,  which  is  gently  drawn  tight  after 
every  completed  stitch  so  that  the  circumference  at 
the  open  end  is  approximately  the  same  as  at  the 
bottom.  The  bag  is  suspended  in  a  jar  of  suitable 
size  by  two  or  three  strings  tied  at  equal  distances  to 
the  string  which  secures  the  circumference.  The  jar 
is  then  slowly  filled  with  water,  while  the  liquid  to  be 
dialysed  is  poured  into  the  bag  at  the  same  time  and 
at  about  the  same  rate,  so  as  to  keep  the  external  and 
internal  level  nearly  the  same  ;  in  this  way  any 
strain  on  the  mouth  of  the  bag  is  avoided  and  it 
retains  its  shape.  The  water  may  be  renewed  from 
time  to  time,  but  it  is  preferable  to  use  a  continuous 
flow,  as  dialysis  is  greatly  accelerated  thereby.  This 


i8 


FIG.  2, 


PARCHMENT  TUBES.  19 

may  be  done  by  allowing  water  to  flow  into  the  outer 
vessel  and  removing  it  by  means  of  a  syphon,  which 
must  be  of  the  type  shown  in  Fig.  3,  to  avoid  either 
the  vessel  or  the  syphon  being  emptied,  if  the  water 
supply  fails  by  any  accident.  The  rate  of  supply 
must,  of  course,  be  so  adjusted  as  not  to  exceed  the 
rate  of  discharge  from  the  syphon,  since  otherwise 
the  water  may  flow  over  the  top  of  the  jar.  It  is 
hardly  necessary  to  add  that  the  same  arrangement 
may  be  used  with  a  Graham  dialyser,  and  also, 
slightly  modified,  with  many  of  the  appliances  yet  to 
be  described.  Continuous  flow  can,  of  course,  be 
used  only  when  the  liquid  remaining  in  the  dialyser 
is  all  that  is  wanted  ;  if  it  is,  for  any  reason,  necessary 
to  examine  the  solution  which  has  diffused  through, 
dialysis  must  be  performed  with  successive  lots 
of  water,  which  may  be  kept  separate  or  be 
combined. 

Parchment  paper  may  also  be  obtained  in  the 
form  of  tubes — "  sausage -skin  dialyser  s  "  as  they  are 
usually  termed  in  catalogues.  They  are  sold  flat, 
and  in  that  condition  are  from  40  to  100  mm.  wide, 
giving  a  diameter,  when  filled  with  liquid,  of  25  to 
70  mm.  As  they  are  easily  damaged,  any  length 
selected  for  use  should  be  carefully  tested  for  leaks. 
It  may  be  used  in  various  ways  :  one  of  the  simplest 
is  to  bend  a  (thoroughly  soaked  and  tested)  piece 
into  U-shape  and  place  it  into  a  tall  cylinder,  allow- 
ing the  open  ends  to  project  an  inch  or  two.  The 
tube  is  then  slowly  filled  with  the  liquid  to  be  dialysed, 
while  the  cylinder  is  at  the  same  time  filled  with 
water  at  about  the  same  rate,  so  that  no  strain  is 
placed  on  the  tube.  Another  method  is  to  close  one 
end  of  the  tube  by  folding  it  over  two  or  three  times, 
the  first  fold  being  about  5  mm.  wide,  and  securing 
this  end  with  a  rubber  clip.  The  clip  is  made  by 
cutting  a  rectangular  strip  about  20  mm.  wide  and 
about  25  mm.  longer  than  the  width  of  the  (flat)  tube 


2O 


FIG.  3. 


PARCHMENT  THIMBLES. 


21 


from  white  rubber  sheet  about  8  to  10  mm.  thick,  in 
which  a  central  straight  incision  is  made  about  5  mm. 
longer  than  the  width  of  the  tube.  This  is  then 
opened  a  little  by  inserting  two  thin  pieces  of  stick 
at  the  ends,  slipped  over  the  folded  end,  and  then 
closed  by  withdrawing  the  sticks.  When  the  use  of 
metal  is  unobjectionable  one  of  the  wire  clips  nsed 
for  attaching  papers  to  one  another  may  be  em- 
ployed, or  a  similar  clip  bent  from  heavy  silver  or 
copper  wire.  The  tube 
should  be  tested  for  leak- 
age after  being  closed. 

Finally,  seamless  thim- 
bles of  parchment  papers 
can  be  obtained,  which, 
although  somewhat  ex- 
pensive, are  reliable  and 
extremely  convenient,  es- 
pecially for  the  examina- 
tion of  small  quantities 
of  liquid.  A  simple 
method  of  using  them  is 
shown  in  Fig.  4.  The 
thimble,  filled  with  the 
solution  to  be  dialysed,  is 


FIG.  4. 


placed  in  an  Erlenmeyer  flask  of  suitable  size  filled 
with  the  solvent.  The  parchment  swells  perceptibly 
in  water,  and  the  neck  of  the  flask  must,  therefore, 
be  a  few  millimetres  larger  in  diameter  than  the 
(dry)  thimble,  to  permit  its  easy  withdrawal  when 
it  is  saturated. 

A  number  of  natural  membranes,  such  as  gold- 
beater's skin,  fish -bladder,  etc.,  have  been  used  for 
dialysis.  Since  they  vary  in  permeability  or  require 
careful  purification*  their  use  can  hardly  be  recom- 
mended except  as  makeshifts,  especially  in  view  of 
the  comparative  ease  with  which  membranes  of  con- 
siderable uniformity  and  covering  a  great  range  of 


22  COLLODION  THIMBLES. 

permeability  can  be  made  by  the  methods  to  be  now 
described. 

These  are  based  on  the  use  of  collodion,  i.e.,  sols 
of  cellulose  nitrate  in  a  mixture  of  ether  and  alcohol. 
The  raw  material  is  obtainable  commercially  as  "  gun 
cotton  "  or  "  pyroxylin,"  and  is  generally  sold  damped 
with  alcohol;  it  should  be  dried  before  weighing. 
The  usual  concentration  is  3  to  4  gm.  of  gun  cotton 
to  100  c.c.  of  ether-alcohol  mixture,  the  proportions 
of  the  latter  varying  between  14  parts  of  alcohol  (90 
per  cent.)  to  86  of  ether,  and  25  parts  of  alcohol  to 
75  parts  of  ether  ;  equal  volumes  of  alcohol  and 
ether  have  also  been  used,  but  this  composition  is 
unusual. 

The  weighed  quantity  of  gun  cotton  is  placed  in  a 
wide-necked  bottle,  the  requisite  volume  of  alcohol 
poured  on  it,  the  bottle  corked  and  allowed  to  stand 
for  about  fifteen  minutes.  The  ether  is  then  added  and 
the  mixture  stirred  occasionally,  until  the  gun  cotton 
has  dissolved ;  it  should  do  so  without  leaving 
any  residue.  The  sol  should  be  almost  clear  and 
does  not  require  filtering.  One  of  the  most  con- 
venient ways  of  employing  it  is  to  make  dialysing 
thimbles  by  coating  the  inside  of  test  tubes  of  suitable 
size  ;  the  beginner  will  find  20  mm.  diameter  x 
125  mm.  long  a  convenient  size,  although  with 
practice  much  larger  thimbles  can  be  made  without 
difficulty.  The  test  tubes  must  be  quite  smooth  on 
the  inside,  thoroughly  clean  and  dry. 

The  selected  test  tube  is  filled  with  collodion,  care 
being  taken  to  pour  it  down  the  side  so  as  not  to  form 
any  air  bubbles.  The  mouth  of  the  test  tube  is  then 
placed  above  that  of  the  bottle  and  the  collodion 
poured  back  slowly  by  slightly  inclining  the  tube  and 
rotating  it  constantly  and  slowly.  The  inclination 
of  the  tube  is  gradually  increased  as  emptying  pro- 
ceeds, but  not  more  than  is  necessary  to  allow  the 
collodion  to  flow  out  in  a  thin  uniform  stream.  If  it 


COLLODION  THIMBLES.  23 

is  raised  too  rapidly  the  bottom  of  the  thimble  is  apt 
to  be  excessively  thin.  The  tube  is  finally  brought 
to  a  vertical  position  and  the  last  remains  of  collodion, 
which  should  not  then  amount  to  more  than  a  few 
drops,  allowed  to  drip  off,  after  which  the  layer  left 
on  the  inside  of  the  tube  is  allowed  to  dry  for  a  short 
time.  Although  the  degree  of  drying  is  the  crucial 
point  of  the  whole  process,  no  definite  rules  can  be 
given  ;  the  collodion  should  not  stick  to  the  finger 
when  touched  lightly,  and  should  just  be  visible  as  a 
faintly  bluish  coating  when  the  tube  is  viewed 
against  a  dark  background.  When  this  stage  is 
reached  the  tube  is  submerged  in  water,  care  being 
taken  to  allow  all  air  to  escape,  and  is  left  for  at  least 
15  minutes.  The  depth  of  water  should  be  about 
2"  more  than  the  diameter  of  the  tube,  so  that  the 
subsequent  operation  can  be  carried  out  without  its 
being  uncovered.  After  the  minimum  time  of 
immersion  has  elapsed,  the  collodion  film  is  detached 
round  the  edge  of  the  tube,  a  finger  inserted  so  as  to 
touch  the  collodion  skin,  and  the  latter  very  slowly 
pulled  out,  while  the  test  tube  is  held  with  the  left 
hand.  It  must  be  remembered  that  the  rate  at 
which  the  collodion  skin  can  be  pulled  out  is  fixed  by 
the  rate  at  which  water  can  flow  through  the  space 
between  it  and  the  wall  of  the  test  tube,  which  is 
necessarily  slow ;  any  attempt  to  hurry  matters  is 
fatal.  If  the  bottom  of  the  vessel  containing  the 
water  is  dark,  the  collodion  membrane  can  be  seen 
very  distinctly,  and  the  bottom  end,rwhich  is  the 
most  likely  portion  to  give  trouble,  watched. 

The  finished  thimbles  can  be  kept  under  water  for 
several  weeks,  undergoing  only  slight  changes  in 
permeability.  On  the  whole,  however,  it  is  advis- 
able to  make  and  use  them  fresh.  They  can  be 
mounted  in  a  variety  of  ways  ;  a  convenient  method 
is  to  insert  a  short  piece  of  glass  tubing,  the  edge  of 
which  has  been  carefully  rounded  in  the  flame,  and 


24  COLLODION  THIMBLES. 

to  fix  the  thimble  to  it  with  collodion,  or  by  tying  ; 
in  the  latter  case  a  strip  of  gutta-percha  tissue  or 
oiled  silk  must  be  wound  over  the  collodion  to 
prevent  it  from  being  cut  by  the  thread  used  for 
tying. 

As  has  already  been  pointed  out,  the  permeability 
of  thimbles  made  from  ether-alcohol  collodion 
depends  very  largely  on  the  extent  of  drying  which 
they  have  undergone  before  immersion.  The  ether 
and  alcohol  still  remaining  in  the  film  is  replaced  by 
water,  and  this  fixes  the  permeability  of  the  hydrogel 
of  cellulose  nitrate  which  ultimately  constitutes  the 
membrane.  Although  practice  soon  enables  a  care- 
ful worker  to  turn  out  fairly  uniform  thimbles,  the 
whole  difficulty  can  be  avoided  by  the  use  of  acetic 
acid  collodion.  This  is  made  by  dissolving  4  gm.  of 
gun  cotton  in  100  c.c.  of  glacial  acetic  acid  ;  lower 
concentrations  give  fragile  films,  while  higher  ones 
produce  unnecessarily  dense  membranes.  The  test 
tubes  are  coated  and  the  excess  emptied  in  exactly 
the  same  way  as  described  ;  the  film  is,  however,  not 
allowed  to  dry,  but  the  coated  tubes  are  immediately 
submerged  in  water.  After  about  30  minutes  they 
may  be  withdrawn  as  explained,  the  operation 
being  generally  easier  than  with  ether-alcohol  collo- 
dion ;  they  are  then  left  in  water,  which  is  occa- 
sionally changed,  until  the  whole  of  the  acetic  acid 
has  diffused  out,  and  may  be  preserved  under  water. 

A  very  convenient  way  of  making  dialysing 
thimbles,  which  are  less  fragile  and  permit  much 
greater  variations  in  permeability  than  those  just 
described,  consists  in  impregnating  the  seamless 
thimbles  of  filter  paper  (Soxhlet  thimbles)  made  in 
various  sizes  for  fat  extraction.  One  of  these  is  held 
vertically  over  a  small  dish  and  filled  to  the  top  with 
collodion  ;  when  the  liquid  has  penetrated  over  the 
entire  surface,  it  is  inverted  and  drained  with  con- 
stant turning.  If  acetic  acid  collodion  is  used,  the 


STAR  DIALYSER.  25 

thimble  is  then  submerged  in  water  immediately  ; 
with  ether- alcohol  collodion  it  must,  like  the  thimbles 
formed  in  test  tubes,  be  allowed  to  dry  for  a  few 
minutes  before  immersion.  Since  the  mechanical 
strength  is  provided  by  the  filter  paper,  collodions 
of  low  concentration  may  be  employed,  2  per  cent, 
in  either  acetic  acid  or  ether-alcohol  being  sufficient 
for  most  purposes.  The  thimbles  are  strong  enough 
to  stand  upright,  and  may  be  used  like  parchment 
paper  thimbles.  A  convenient  method  of  using  any 
type  of  thimble  is  to  stand  or  suspend  it  in  a  cylin- 
drical vessel  of  slightly  larger  diameter,  provided 
with  an  inlet  at  the  bottom  and  an  overflow  outlet  at 
a  level  i  to  2  cm.  below  the  top  edge  of  the  thimble. 
Water  is  continuously  passed  in  at  the  bottom  and 
overflows  at  the  top,  and  dialysis  proceeds  with  great 
rapidity  with  comparatively  small  quantities  of  water. 
Flat  membranes  of  (ether-alcohol)  collodion  are 
rather  easier  to  make  than  thimbles,  and  can  con- 
veniently be  used  for  continuous  dialysis  in  the  "  Star 
Dialyser  "  described  by  Zsigmondy.  The  apparatus 
(Fig.  5)  consists  of  two  parts,  both  of  ebonite,  a  disc 
provided  with  a  rim  about  10  to  15  mm.  deep,  and  a 
cylinder  which  fits  loosely  into  the  latter,  open  at  both 
ends  and  30  to  40  mm.  deep.  The  disc  has  a  central 
inlet  and  its  upper  face  is  provided  with  six  or  eight 
ribs,  about  3  mm.  deep,  which  stop  a  few  millimetres 
short  of  both  the  central  opening  and  of  the  rim. 
To  prepare  the  membrane  the  ring  is  placed  on  a 
clean  piece  of  plate  glass  and  sufficient  collodion 
poured  into  it  to  cover  the  glass  to  a  depth  of  2  or 
3  mm.  The  ring  is  lifted  slightly,  to  allow  the 
collodion  to  penetrate  between  it  and  the  glass ; 
to  strengthen  the  joint  thus  made,  the  outside  of 
the  ring  is  painted  with  collodion  to  a  height  of 
about  5  mm.  from  its  lower  edge.  After  the  collo- 
dion has  dried  some  minutes  water  is  poured  into 
the  ring,  which,  together  with  the  collodion  mem- 


26 


STAR  DIALYSER. 


brane  adhering  to  it,   can  be  lifted  off  the  glass 
after  about  10  or   15  minutes.     The  ring  is  then 


!                 1 

*l///////////////////tfff/f/h 

1 

Y////////////////////////////* 

T 

FIG.  5. 


placed  into  the  disc,  filled  with  liquid  to  be  dialysed, 
and  water  is  passed  through  the  central  inlet,  which 


SPECIAL  DIALYSERS.  27 


n 


overflows  round  the  edge  of  the  rim.  As  it  is  diffi- 
cult to  adjust  the  apparatus  so  exactly  that  overflow 
is  uniform  all  round  the  rim,  it  is  best 
to  localize  it  by  means  of  two  or  three 
strips  of  filter  paper,  placed  between 
the  open  cylinder  and  the  rim  and  bent 
over  the  latter,  so  as  to  act  as  syphons. 
Special  arrangements  are  necessary 
when  the  sol  to  be  dialysed  has  an 
appreciable  osmotic  pressure,  as  is  the 
case,  e.g.,  with  albumin  sols.  In  this 
case  water  flows  into  the  dialyser, 
diluting  the  sol  and  eventually  causing 
it  to  overflow.  The  only  way  to  prevent 
this  is  to  counterbalance  the  osmotic 
pressure  hydrostatically ;  in  other 
words,  to  keep  the  level  of  the  sol  in 
the  dialyser  above  the  water  level  out- 
side from  the  beginning.  For  small 
quantities,  such  as  come  into  question 
here,  the  simplest  arrangement  is  that 
shown  in  Fig.  6.  A  dialysing  thimble 
of  either  parchment  or  paper  impreg- 
nated with  collodion  is  fitted  with  a 
rubber  stopper  and  tied  tightly  ;  a 
strip  of  gutta-percha  tissue  about 
15  mm.  wide  is  first  wound  round  the 
end  of  the  thimble  and  strong  thread 
tied  over  this.  Through  the  rubber 
stopper  passes  a  funnel  tube  about  30 
cm.  long,  which  must  have  a  diameter 
of  at  least  8  mm.,  so  that  the  sol  can 
be  poured  down  one  side  of  it,  allowing 
the  air  to  escape  and  the  thimble  and 
tube  to  be  filled  to  the  top.  The 
thimble  is  submerged  in  a  beaker 
through  which  water  flows  con- 
tinuously. With  this  arrangement  FIG.  6. 


28  DIALYSERS. 

the  liquid  remains  at  its  original  concentration  and 
the  bulk  of  it  is  still  contained  in  the  dialysing 
membrane,  as  the  volume  of  the  funnel  tube  is 
comparatively  small. 

LITERATURE. 

Membranes  made  from  collodion  of  unusually  high 
concentration  and  capable  of  standing  considerable 
pressures  are  described  by  A.  T.  Glenny  and  G.  S. 
Walpole,  Biochem.  Journ.,  IX.,  284  (1915) ;  G.  Wegelin, 
Koll.-Zeitschr.,  XVIIL,  225  (1916),  a  new  method  of 
rapid  dialysis  and  ultra-filtration. 


CHAPTER  III. 

SUSPENSOID  SOLS. 

A.    METALLIC  SOLS. 

Gold  Sols. — A  i  per  cent,  solution  of  gold  chloride 
[more  correctly,  auro-chlorhydric  acid,  HAuCl4 . 
3H2O)  serves  as  the  starting  material.  The  "  gold 
chloride  "  of  photography,  NaAuCl^  .  2H2O,  may  be 
used  instead  for  all  the  methods,  with  the  exception 
of  Zsigmondy's  ;  a  i  per  cent,  solution  of  this  salt  is 
obtainable  in  commerce.  For  the  method  first 
described,  reduction  by  tannin,  the  gold  chloride 
must  be  made  exactly  neutral  to  litmus  by  addition 
of  sodium  or  potassium  carbonate  (N/5  solution). 

Reduction  by  tannin  (Wo.  Ostwald).  Dissolve 
o-i  gm.  of  purest  tannin  in  100  c.c.  of  water.  If  this 
solution  is  to  be  kept  it  should  receive  an  addition 
of  a  few  drops  of  chloroform,  without  which  it  goes 
mouldy. 

Dilute  i  c.c.  of  the  gold  chloride  solution  with 
200  c.c.  of  water,  stir  and  add  i  c.c.  of  the  tannin 
solution,  then  warm  over  a  Bunsen  burner.  Reduc- 
tion gradually  proceeds  and  the  liquid  becomes  red. 
Continue  heating  and,  when  the  liquid  boils,  add 
another  cubic  centimetre  of  gold  chloride  solution, 
followed  by  a  cubic  centimetre  of  tannin.  The 
resulting  sol  should  be  perfectly  clear  in  transmitted 
light  and  of  deep  ruby -red  colour.  The  mixture 
must  be  well  stirred  after  every  addition. 

Sometimes  the  colour  of  the  liquid  containing  the 
first  lot  of  gold  chloride  and  tannin  does  not  become 
red  while  warming,  but  purple,  or  even  a  cold  violet. 


30  METALLIC  SOLS. 

Do  not  be  deterred,  but  continue  to  heat  to  boiling  ; 
after  the  second  addition  of  gold  chloride  and  tannin 
at  boiling  point  the  colour  very  generally  changes  to 
red  without  even  a  tinge  of  purple. 

Reduction  may  also  be  carried  out  in  the  cold  by 
using  a  larger  proportion  of  tannin  solution,  say 
100  c.c.  of  water,  i  c.c.  of  gold  chloride  solution,  and 
3  to  5  c.c.  of  tannin  solution,  added  gradually.  In 
this  case  the  sol  is  more  liable  to  have  a  purple  or 
bluish  tinge. 

Sols  made  by  these  methods  are  liable  to  the 
growth  of  mould  on  keeping  and  gradually  lose 
colour,  the  gold  being  deposited  on  the  mycelium  of 
the  mould.  This  trouble  may  be  prevented  by 
adding  a  few  drops  of  chloroform,  or,  in  view  of  the 
great  simplicity  of  the  method,  by  preparing  the  sols 
when  and  as  required.  These  sols  are  protected  to 
a  slight  and  uncertain  extent  by  the  tannin  and  its 
oxidation  products,  and  are  less  suitable  for  coagula- 
tion experiments  than  the  following. 

Reduction  by  formaldehyde  (R.  Zsigmondy) .  Heat 
120  to  150  c.c.  of  redistilled  water  in  a  3oo-c.c. 
beaker ;  while  it  is  warming  add  i  c.c.  of  gold 
chloride  solution  (i  per  cent.),  and  then  2*5  to  3  c.c. 
of  a  N/5  solution  of  purest  potassium  carbonate.  As 
soon  as  the  solution  boils  stir  vigorously  ;  add 
gradually,  but  fairly  quickly,  2  to  3  c.c.  of  dilute 
formaldehyde  solution  (i  c.c.  of  commercial  40  per 
cent,  formalin  to  100  c.c.  of  water)  and  extinguish 
the  flame.  Reduction  is  complete  in  about  a 
minute,  and  the  resulting  sol  should  be  perfectly 
clear  in  transmitted  light  and  of  pure  ruby-red 
colour  without  purple  tinge. 

The  beaker  used  should  be  of  resistance  glass,  and 
stirring  rods  of  the  ordinary  soft  glass  must  not  be 
used ;  a  tube  of  resistance  glass  closed  at  one  end 
should  be  used  for  stirring.  The  sol  should  also  be 
kept  in  vessels  of  resistance  glass. 


METALLIC  SOLS.  31 

For  some  reason,  which  is  still  obscure,  larger 
batches  than  about  150  c.c.  cannot  be  made  successfully. 
If  larger  quantities  are  required,  they  must  be  made 
in  150  c.c.  lots  as  described  ;  since  all  the  solutions 
can  be  made  up  in  large  quantities  and  keep  indefi- 
nitely, there  is  no  difficulty  in  preparing  any  volume 
of  sol  likely  to  be  required. 

The  sol  can  be  dialysed  against  redistilled  water, 
but  will  keep  without  this  being  done  in  vessels  of 
resistance  glass.  It  is  very  suitable  as  a  standard 
preparation  for  experiments  on  electrolyte  coagu- 
lation, protection,  etc. 

Palladium  Sol. — This  can  be  prepared  by  exactly 
the  same  procedure  as  Zsigmondy's  gold  sol,  using 
the  following  quantities  :  150  c.c.  of  water,  i  c.c.  of 
i  per  cent,  palladium  chloride  solution,  and  0*4  c.c. 
of  N/io  sodium  carbonate  solution,  reduced  by  3  to 
4  c.c.  of  dilute  formaldehyde  (i  c.c.  of  commercial 
formalin  to  100  c.c.  of  water).  The  sol  should  be 
brown  and  perfectly  clear  in  transmitted  light. 

Silver  Sol. — Reduction  by  dextrine  (Carey  Lea's 
method).  This  is  one  of  the  best  examples  of  a 
highly  concentrated  metallic  sol,  as  concentrations 
up  to  5  per  cent,  of  Ag  can  be  obtained  in  favourable 
conditions.  The  following  quantities  should  be  tried 
first,  but  there  is  no  difficulty  in  dealing  with  four  or 
five  times  these  amounts. 

Dissolve  4  gm.  of  commercial  dextrine  in  100  c.c. 
of  water  and  then  4  gm.  of  purest  caustic  soda. 
Dissolve  3  gm.  of  silver  nitrate  in  20  c.c.  of  water 
and  add  to  the  dextrine-soda  solution.  A  precipi- 
tate of  silver  oxide  forms,  which  is  gradually  reduced 
by  the  dextrine,  the  colour  changing  to  a  reddish- 
brown.  Allow  20  to  30  minutes  for  this,  and  then 
add  100  c.c.  of  96  per  cent,  alcohol  and  stir.  Allow 
the  mixture  to  settle  for  another  15  to  20  minutes, 
and  then  pour  off  the  turbid  liquid  from  the  sediment 
of  silver  as  completely  as  possible.  On  pouring  on 


32  METALLIC  SOLS. 

water  the  silver  generally  disperses  immediately  ; 
should  this  not  be  the  case,  a  little  shaking  and 
stirring  will  be  sufficient  to  induce  dispersion. 

The  silver  amounts  to  1-81  gm.,  and  in  favourable 
conditions  35  to  40  c.c.  of  water  will  disperse  the 
whole  of  it,  so  that  the  sol  contains  about  5  per  cent, 
of  disperse  phase.  It  is,  however,  advisable  to  use 
a  greater  volume  of  water,  say  about  180  c.c.  This 
sol  is  dark  brown  and  opaque  even  in  thin  layers  ; 
when  diluted  with  about  50  times  the  amount  of 
water  it  should  be  clear  in  transmitted  light,  with  a 
greenish-black  surface  colour  in  reflected  light.  The 
colour  of  the  sol,  and  in  fact  the  success  of  the  whole 
method,  depends  a  good  deal  on  the  quality  of  the 
dextrine,  which  can  be  determined  only  by  experi- 
ment. Generally  speaking,  the  ordinary  yellow 
commercial  brands  work  better  than  a  highly  purified 
product.  The  i  per  cent,  sol  may  be  kept  unaltered 
for  a  long  time. 

Reduction  by  tannin.  Add  to  100  c.c.  of  water 
I  c.c.  of  I  per  cent,  silver  nitrate  solution,  and  then 
a  few  drops  of  weak  ammonia.  Reduce  with  3  to 
4  c.c.  of  0-5  per  cent,  tannin  solution.  The  sol  should 
be  brown  and  perfectly  clear  in  transmitted  light, 
with  a  marked  green  surface  colour  in  reflected  light. 

Reduction  by  hydrogen  (Kohlschuetter's  method). 
This  process  is  of  interest  as  giving  an  electrolyte- 
free  sol.  Dissolve  i  gm.  of  silver  nitrate  in  20  c.c. 
of  water  and  precipitate  with  a  slight  excess  of 
caustic  soda.  Wash  the  precipitate  of  silver  oxide 
by  repeated  decantation  with  hot  water,  and  then 
suspend  it  in  200  c.c.  of  redistilled  water,  shake  well, 
and  then  filter  off  any  undissolved  oxide.  Pour  the 
solution  into  a  resistance  glass  flask  kept  at 
56°  to  60°  C.  in  a  water  bath  (or  thermostat),  and 
pass  a  current  of  hydrogen  through  it  by  means  of 
a  tube  of  resistance  glass.  Reduction  is  complete  i:i 
20  to  25  minutes. 


SULPHIDE  SOLS.  33 

Other  methods.  As  gold  chloride  is  reduced  by 
most  reducing  agents,  a  very  large  number  of 
methods  of  preparation  of  gold  sols  are  possible ; 
references  to  some  of  these  are  given  at  the  end  of 
this  chapter.  Generally  speaking,  sols  will  result  if 
gold  chloride  solutions  containing  about  one  part  in 
10,000  are  treated  with  small  quantities  of  the 
following  reducing  agents,  in  solutions  containing 
from  one  part  in  4,000  to  one  in  500  :  gallic  acid, 
hydroquinone,  pyrocatechin,  white  phosphorus  in 
ether  (Faraday's  method,  developed  by  Zsigmondy, 
q.v.},  hydrazine  hydrate  (Gutbier),  phenylhydrazine 
hydrochloride  (Gutbier  and  Resenscheck) ,  etc.  Silver 
may  similarly  be  reduced  from  dilute  silver  nitrate 
by  ferrous  citrate  (Carey  Lea),  in  alkaline  solution 
by  hydrazine  hydrate,  all  photographic  developers, 
etc. 

B.    SULPHIDE  SOLS. 

Cadmium  Sulphide  Sol. — This  is  an  instance  of  a 
sol  produced  by  peptisation  of  a  coarse  precipitate. 

Dissolve  0-5  gm.  of  cadmium  chloride  in  20  c.c.  of 
water  and  precipitate  with  moderately  concentrated 
ammonium  sulphide.  The  precipitate  should  be  a 
deep  yellow  and  should  settle  rapidly ;  if  it  does  not, 
the  ammonium  sulphide  solution  requires  diluting. 
Wash  the  precipitate  by  decantation  with  two  or 
three  lots  of  water,  50  c.c.  each,  and  suspend  in  300 
to  400  c.c.  of  water.  Pass  a  slow  stream  of  hydrogen 
sulphide  through  the  mixture  and  shake  occasion- 
ally. The  suspension  first  becomes  milky,  then 
yellow  and  moderately  clear,  and  after  20  to  25 
minutes  most  of  the  precipitate  will  have  been  dis- 
persed. The  sol  may  be  filtered  to  removeTany 
remains  of  precipitate,  and  boiled  to  drive  off  the 
excess  of  hydrogen  sulphide,  without  coagulation 
occurring.  The  filtered  sol  is  a  pale  golden  yellow 


34  MISCELLANEOUS   SOLS. 

in  transmitted  light,  with  marked  greenish  opales- 
cence  in  reflected  light. 

Arsenic  Sulphide  Sol. — This  sol  has  been  the  sub- 
ject of  many  classical  investigations,  especially  on 
electrolyte  coagulation.  To  prepare  it,  dissolve 
2  gm.  of  arsenic  trioxide  in  one  litre  of  water  ;  keep 
the  latter  boiling  until  solution  is  complete.  After 
cooling  the  liquid  pass  a  slow  stream  of  hydrogen 
sulphide  through  it,  with  occasional  stirring,  until  the 
colour  does  not  deepen  perceptibly.  The  sol  is  a 
pale  orange  colour  in  transmitted  light,  with  a 
greenish-yellow  opalescence  in  reflected  light.  Excess 
of  H2S  can  be  removed  by  passing  hydrogen  through 
the  sol ;  this  must  be  done  if  the  sol  is  to  be  used  for 
coagulation  experiments. 


C.    MISCELLANEOUS  PREPARATIONS. 

Prussian  Blue  Sol. — Dissolve  0-4  gm.  of  crystal- 
lized potassium  ferrocyanide  in  20  c.c.  of  water  and 
0-4  gm.  of  ferric  chloride  in  20  c.c.  of  water.  Pour 
the  first  solution  into  the  second,  slowly  and  without 
stirring.  Allow  the  mixture  to  stand  for  a  few 
minutes  and  then  pour  it  on  a  folded  filter  of  hard 

Eaper.     The  filtrate  should  be  quite  clear  and  run 
drly  freely.     When  filtration  is  complete,  wash  the 
precipitate  with  four  successive  lots  of  25  c.c.  of 
water. 

The  precipitate  is  then  dissolved  in  300  c.c.  of 
solution  containing  16  gm.  of  crystallized  oxalic  acid. 
The  simplest  way  to  do  this  is  to  pour  the  acid 
solution  on  the  filter  and  allow  it  to  percolate  ;  the 
precipitate  will  be  found  to  have  been  completely 
dissolved  when  the  whole  volume  of  acid  has  passed 
through  the  filter.  The  solution  of  Prussian  blue  in 
oxalic  acid  is  then  dialysed  in  a  parchment  bag 
against  repeated  changes  of  distilled  water,  until  the 


FERRIC  HYDROXIDE   SOL.  35 

last  batch  of  the  latter  gives  no  perceptible  oxalate 
reaction.  Owing  to  its  deep  colour  and  sensitiveness 
the  sol  is  very  suitable  for  cataphoresis  and  coagula- 
tion experiments.  For  the  latter  purpose  the  sol 
can  be  diluted  with  an  equal  volume  of  water,  as 
even  in  that  dilution  it  is  deeply  coloured  in  a  thick- 
ness of  i  cm.  The  concentrated  sol  is  quite  stable, 
and  there  is,  therefore,  no  reason  for  making  a  dilute 
sol  directly,  as  this  course  entails  waste  of  oxalic 
acid. 

Ferric  Hydroxide  Sol.* — Heat  500  c.c.  of  water  in 
a  tall  beaker  and,  when  it  is  boiling  vigorously,  add 
2  c.c.  of  a  30  per  cent,  solution  of  ferric  chloride, 
gradually  and  with  stirring.  The  liquid  turns  a 
deep  reddish-brown  and  remains  perfectly  clear. 

The  sol  contains  HC1,  corresponding  to  0-6  gm.  of 
FeCl3,  i.e.,  about  0*4  gm.,  or  approximately  22  milli- 
moles  per  litre.  As  this  is  a  small  fraction  only  of  the 
HC1  concentration  required  for  coagulation,  the  sol 
may  be  used  for  precipitation  experiments  without 
being  dialysed,  as  well  as  for  cataphoresis  in  the 
U-tube  (no  particles  are  visible  with  dark-ground 
illumination,  so  that  the  microscopic  method  is  not 
applicable).  Most  of  the  HC1  can  be  removed  by 
dialysis  in  the  parchment  bag,  but  only  experience 
will  tell  how  far  dialysis  may  be  continued  without 
coagulation  of  the  sol.  Both  the  acid  and  the 
dialysed  sol  keep  indefinitely.  Only  the  latter  is 
suitable  for  experiments  on  the  mutual  coagulation 
of  oppositely  charged  sols. 

LITERATURE 

For  all  inorganic  suspensoid  sols  :  Th.  Svedberg,  "  Die 
Methoden  zur  Herstellung  kolloider  Loesungen  anorgani- 

*  Although  this  sol  has  some  emulsoid  properties,  it  is  classed 
here  with  the  suspensoids  on  account  of  its  behaviour  to  elec- 
trolytes, etc. 


36  INORGANIC  SOLS. 

scher  Stoffe,"  Theodor  Steinkopff,  Dresden,  1909.  Sols 
with  unusual  electric  charges  :  H.  S.  Long,  Proc.  Univ. 
of  Durham  Phil.  Soc.,  V.,  Part  2  (1913),  positive  red  gold 
sol;  F.  Powis,  Journ.  Chem.  Soc.,  107,  818  (1915), 
negative  ferric  hydroxide  sol.  Dye  sol  with  charac- 
teristic suspensoid  properties  :  Wo.  Ostwald,  Kolloid- 
chemische  Studien  am  Kongorubin,  Roll.  Beihefte,  X.,  179 
(1919). 


CHAPTER  IV. 
SUSPENSIONS. 

Mastic  Suspension. — This  preparation  is  one  of  the 
classical  subjects  of  investigation.  Dissolve  o-i  gm. 
of  powdered  gum  mastic  in  10  c.c.  of  alcohol  or 
acetone.  Pour  the  solution  slowly  into  500  c.c.  of 
water,  stirring  the  latter  vigorously.  Filter  the 
suspension  through  a  fairly  close  filter  paper  to 
remove  coarser  particles. 

The  preparation  is  almost  opaque,  with  vivid  pale 
blue  opalescence,  in  reflected  light,  and  should  be 
perfectly  clear  and  a  faint  yellow  in  transmitted 
light.  It  shows  an  extremely  bright  blue  Tyndall 
cone. 

Coagulation  experiments  should  be  made  with 
HC1  and  with  salts  of  uni-,  bi-  and  tri-valent  cations, 
as  the  suspension  behaves  somewhat  differently 
from  suspensoid  sols  (see  chapter  on  Electrolyte 
Coagulation).  "  Titration  "  will  be  found  somewhat 
difficult,  as  there  is  no  marked  sudden  change ;  on 
standing,  however,  the  disperse  phase  separates  very 
clearly  as  a  flocculent  precipitate,  though  sedimenta- 
tion is  naturally  slow.  In  the  U-tube  a  sharp  boun- 
dary will  be  seen  if  observed  in  reflected  light. 

A  similar  suspension,  using  exactly  the  same 
quantities,  may  be  made  from  other  resins.  The 
beginner  will  find  dragon's  blood  convenient,  as  the 
colour  is  a  vivid  red. 

Gamboge  may  be  treated  in  the  same  way.  A 
suspension  which  shows  the  Brownian  movement, 
cataphoresis  under  the  microscope,  etc.,  can  also  be 


38  SUSPENSIONS. 

made  by  rubbing  down  a  stick  of  the  gum  with  a 
few  cubic  centimetres  of  water  in  a  saucer  (as  is  done 
with  sticks  of  Chinese  ink),  diluting  the  resulting 
mixture  with  a  large  volume  of  water  and  filtering  to 
remove  coarser  particles. 


CHAPTER  V. 
ORGANOSOLS. 

THE  most  convenient  method  of  directly  preparing 
organosols  of  the  noble  metals  is  that  of  C.  Amberger, 
in  which  wool-fat  (lanoline)  is  used  as  protective 
agent. 

To  prepare  silver  sol,  dissolve  3-5  gm.  of  silver 
nitrate  in  5  c.c.  of  water  and  add  this  solution  in  very 
small  quantities  at  a  time  to  15  gm.  of  cold  lanoline, 
incorporating  it  thoroughly  with  the  latter  by  means 
of  a  pestle  or  a  silver  spatula.     The  success  of  the 
subsequent  reduction  depends  on  the  completeness 
with  which  this  is  done.     If  any  silver  nitrate  is  left 
in  the  form  of  drops,  the  oxide  and  silver  formed 
from  them  are  of  course  not  protected  by  the  wool-fat 
and  remain  as  a  coarse  insoluble  residue  when  the 
latter  is  taken  up  in  an  organic  solvent.     Then  add 
in  the  same  way  a  solution  of  i  gm.  of  sodium 
hydroxide  in  5  c.c.  of  water.     The  mass  turns  first 
yellow  and  then  brown,  owing  to  the  formation  of 
silver  oxide.     On  standing  in  the  light  the  latter  is 
reduced  to  silver ;    the  reduction  is  accelerated  by 
gentle  warming  and  by  turning  over  the  mixture 
from  time  to  time,  so  as  to  expose  the  whole  of  it  to 
the  light.     After  about  six  hours  reduction  is  gene- 
rally complete,  and  the  product  is  dissolved  in  50  c.c. 
of  chloroform.     Fifty  c.c.  of  petroleum  ether  and 
about  25  gm.  of  fresh  granulated  calcium  chloride 
are  then  added — the  latter  to  remove  water,  etc. — 
and  the  mixture  allowed  to  stand  for  five  to  six  hours. 
The  solution,  which  should  be  a  clear  reddish-brown 


40  ORGANOSOLS. 

when  diluted  with  about  20  volumes  of  solvent,  is 
then  poured  off ;  the  solvent  may  be  allowed  to 
evaporate,  leaving  a  mass  of  colloidal  silver  in  wool- 
fat  of  the  original  salve-like  consistency.  This  dis- 
solves easily  in  ether,  petroleum  ether,  also  in  fatty 
oils  and  in  paraffin. 

Organosols  of  gold,  platinum  and  metals  of  the 
platinum  ^roup  may  be  prepared  in  similar  fashion, 
for  which  the  original  papers  should  be  consulted. 

LITERATURE. 

C.  Amberger,  Koll.-Zeitschr.,  XL,  97, 100  (1912),  silver 
and  gold ;  XIIL,  310,  313  (1913)  ;  XVIL,  47  (1915), 
platinum  and  metals  of  the  platinum  group. 


CHAPTER  VI. 
EMULSOID  SOLS  AND  GELS. 

A.    SILICIC  ACID  SOL  AND  GEL. 

A  CONVENIENT  starting  material  is  a  solution  of 
sodium  silicate  having  a  density  i«i6,  made  by 
diluting  the  commercial  water-glass  syrup  with 
freshly  boiled  distilled  water.  The  ratio  of  syrup  to 
water  is  best  ascertained  by  preparing  a  small  lot, 
sufficient  for  determining  the  density  with  a  spindle. 
The  solution  may  be  prepared  in  large  quantities, 
and  should  be  kept  in  a  bottle  closed  by  a  rubber 
stopper  or  a  glass  stopper  well  rubbed  with  vaseline. 

To  prepare  a  sol,  dilute  30  c.c.  of  concentrated 
hydrochloric  acid  (1*2  sp.  gr.)  with  100  c.c.  of  water, 
and  pour  75  c.c.  of  the  sodium  silicate  solution  into 
the  dilute  acid.  The  mixture  is  dialysed  in  a  parch- 
ment bag  against  repeated  changes  or  against 
running  water ;  the  beginner  will  find  the  former 
course  more  satisfactory.  Experience  will  show  how 
far  it  is  possible  to  push  dialysis  without  the  sol 
setting  to  gel  prematurely  in  the  dialyser. 

The  sol  should  be  perfectly  clear  and  colourless. 
It  will  keep  for  a  length  of  time  which  can  be  ascer- 
tained only  by  experience ;  as  the  removal  of  gel, 
formed  accidentally,  from  flasks  or  bottles  with  nar- 
row necks  is  inconvenient,  sol  under  examination 
should  be  kept  in  wide-mouthed  bottles  or  taper 
beakers. 

Setting  is  greatly  accelerated  by  C02,  carbonates, 
phosphates,  and  free  alkali.  The  effect  can  be 
demonstrated  by  bubbling  C02  gas  through  the  sol 


42  SILICIC  ACID  SOL. 

until  the  bluish  tinge,  which  indicates  the  beginning 
of  gelation,  appears  ;  or  by  adding  small  amounts  of 
dilute  solutions  of  carbonate,  phosphate  or  ammonia 
to  the  sol,  gradually  and  with  constant  stirring, 
which  is  discontinued  as  soon  as  the  sol  appears 
bluish.  If  the  solutions  are  too  concentrated,  or  are 
added  too  rapidly,  local  coagulation  and  flocculation 
may  occur  instead  of  complete  gelation. 

The  concentrations  given  above  are  fairly  high 
and  will  be  found  useful  if  a  stiff  gel  is  required.  If 
the  sol  alone  is  wanted  and  requires  keeping  for  some 
time,  the  same  quantities  of  hydrochloric  acid  and 
of  silicate  solution  should  be  used,  but  a  larger 
volume  of  water. 

To  determine  the  amount  of  Si02  in  a  given  sol, 
evaporate  5  c.c.  slowly  in  a  weighed  crucible  to  dry- 
ness  and  then  ignite  until  the  weight  is  constant. 
In  the  later  stages  of  drying  gelation  may  occur,  and 
the  steam  bubbles  formed  in  the  gel  burst  violently 
and  may  scatter  some  of  the  material,  unless  drying 
proceeds  very  slowly. 

The  effect  of  lyo tropic  additions  is  the  same  as  in 
the  case  of  other  emulsoid  sols.  This  can  be  shown 
qualitatively  by  placing  10  c.c.  of  freshly  dialysed 
sol  in  each  of  three  test  tubes,  keeping  one  as  blank 
and  saturating  the  others  respectively  with  sodium 
sulphate  and  with  ammonium  thiocyanate.  The 
sol  containing  Na2S04  will  set  before,  and  that  con- 
taining NH4CNS  after,  the  blank  sample  ;  the  latter 
very  generally  does  not  set  at  all. 

All  vessels,  measures,  etc.,  used  for  sodium  silicate 
or  silicic  acid  sol  should  be  washed  immediately  and 
thoroughly. 

B.    GELATIN  AND  AGAR  SOLS  AND  GELS. 

Gelatin  occurs  in  commerce  as  "  leaf  "  gelatin  in 
sheets  about  9"  to  10"  long  by  4"  to  5"  wide,  showing 


GELATIN   SOL.  43 

the  diamond-shaped  marks  of  the  wire  netting  on 
which  the  leaf  has  been  dried  ;  as  powder,  and  as  foil 
of  uniform  thickness — about  0*15  mm. — without  any 
marks.  The  most  suitable  brands  for  practically  all 
the  work  to  be  described  are  Coignet's  "  Photo- 
graphic "  and  "  First  Quality/'  and  Nelson's  "  Crystal 
Leaf."  Since  different  brands  differ  appreciably 
in  their  physical  constants  and  in  their  ash  content, 
it  is  essential  to  start  any  given  investigation  with 
an  amply  sufficient  stock  of  the  brand  selected.  If 
great  constancy  is  aimed  at,  it  is  desirable  to  take 
leaves  at  random  throughout  a  one-pound  package, 
or  to  shear  through  the  entire  package  and  use  the 
strips  so  obtained  rather  than  the  necessary  number 
of  adjacent  leaves. 

For  many  purposes,  i.e.,  in  all  cases  in  which  only 
reproducible  and  not  quantitative  results  are  aimed 
at,  sols  and  gels  containing  a  definite  amount  to  a 
given  volume  of  water,  e.g.,  10  gm.  of  gelatin  to 
100  c.c.  of  water,  are  quite  suitable  and  are  easier  to 
prepare  than  sols  containing  a  specified  amount  in  a 
definite  volume  of  sol.  The  leaf  is  broken  into  pieces 
preferably  not  larger  than  £"  square,  placed  in  a 
beaker,  and  the  requisite  amount  of  water  poured  on, 
care  being  taken  that  the  whole  of  the  leaf  is  covered  ; 
air  bubbles  should  be  removed  by  shaking  or  stirring. 
The  gelatin  is  then  allowed  to  swell,  either  to  com- 
plete saturation,  or  for  any  arbitrarily  fixed  period, 
which,  however,  should  not  be  less  than  two  or  three 
hours.  Complete  swelling  may  take  24  hours  or 
even  more ;  as  gelatin  imbibes  something  like  ten 
times  its  weight  of  water,  there  will  be  no  loose  or 
unimbibed  water,  if  the  amount  originally  put  on 
was  less  than  ten  times  the  weight  of  gelatin  leaf. 
The  thickened  edges  of  the  leaf  take  considerably 
longer  to  swell  than  the  rest,  and  care  should  be 
taken  that  the  time  allowed  is  sufficient  to  soften 
them  completely. 


44  GELATIN   SOL. 

The  next  operation  is  the  dispersing  of  the  gelatin, 
which  should  be  carried  out  on  the  water  bath.  A 
temperature  of  35°  to  45°  C.  is  sufficient,  but  higher 
temperatures  may  be  used  to  accelerate  the  process 
and  for  other  reasons.  Thus,  if  the  sol  is  to  be 
filtered  (see  below) ,  it  will  be  advisable  to  heat  up  to 
80°  or  90°,  as  otherwise  the  viscosity  is  high  and  the 
rate  of  nitration  excessively  low.  It  is  necessary  to 
bear  in  mind  that  the  properties  of  a  gelatin  gel  or  sol 
are  not  merely  functions  of  the  concentration  and  tem- 
perature, but  depend  on  its  whole  previous  history,  viz., 
the  period  allowed  for  swelling,  the  temperature  at  which 
the  sol  was  formed  and  the  length  of  time  during  which 
it  was  exposed  to  this  temperature.  To  eliminate 
differences  in  the  history,  the  practice  is  sometimes 
adopted  of  heating  the  sols  for  a  definite  time,  say 
five  minutes,  to  100°,  cooling  at  a  definite  rate  to  the 
temperature  at  which  the  sol  is  to  be  used  (e.g.,  for 
viscosity  measurements)  and  keeping  the  sol  at  the 
lower  temperature,  likewise  for  a  definite  time, 
before  use.  While  this  treatment  goes  a  considerable 
way  towards  obliterating  the  "  thermal  history,"  it 
is  yet  safer  to  adopt  a  rigidly  uniform  procedure  in 
any  particular  investigation. 

The  sol  in  many  cases  does  not  require  filtration 
and  is  ready  for  use  when  the  gelatin  is  completely 
dispersed.  If  the  gel  is  wanted  especially  for  the 
study  of  its  elastic  or  optical  properties,  it  must  not 
be  used  for  at  least  four  hours  after  setting  is  appa- 
rently complete,  as  the  modulus  and  the  accidental 
birefringence  do  not  attain  their  final  values  before 
that  time.  Bodies  of  gel  of  definite  shape  can  be 
made  by  pouring  the  sol  into  suitable  moulds ;  thus 
cylinders  can  be  made  by  using  glass  or  metal  tubes 
closed  at  one  end,  from  which  the  gel  cylinder  is 
removed  by  dipping  them  into  boiling  water  and 
allowing  the  gel  to  drop  into  an  ample  depth  of  cold 
water.  Other  shapes,  e.g.,  prismatic  ones,  can  be 


GELATIN  SOL.  45 

cast  in  moulds  made  from  heavy  tin  or  lead  foil,  or 
wooden  moulds  lined  with  ordinary  tin  foil,  which  is 
rubbed  with  vaseline,  any  excess  being  removed  by 
wiping  with  cotton-wool.  In  all  cases  the  gel  should 
be  left  in  the  mould  for  several  hours  after  setting, 
as  mentioned  above. 

In  many  cases,  and  always  when  a  salt  capable  of 
forming  a  precipitate  with  calcium  salts,  chlorides, 
sulphites  or  sulphates  has  to  be  added,  the  sol  will 
require  filtering.  The  most  suitable  paper  is  Char  din's, 
either  the  original  brand  or  an  imitation  made  in 
England.  It  can  be  obtained  in  sheets  or  as  folded 
filters,  which,  however,  are  too  large  for  the  small 
batches  usually  required.  Folded  filters  should  be 
made,  great  care  being  taken  with  the  point ;  as  the 
paper  is  rather  thick  it  is  not  advisable  to  try  to  make 
more  than  twelve  folds.  A  hot-water  funnel  is 
used  ;  those  usually  obtainable  have  the  defect  that 
the  spout  of  the  glass  funnel  is  much  too  long,  so  that 
cooling  and  even  setting  may  take  place  in  the  por- 
tion which  passes  through  the  stopper  of  the  water- 
jacket.  To  obviate  this,  a  rather  thin  stopper,  not 
more  than  £",  should  be  used,  and  the  spout  of  the 
glass  funnel  cut  off  so  as  just  to  project  through  the 
stopper.  The  temperature  of  the  water  bath  should 
not  be  higher  than  is  necessary  to  secure  a  reasonable 
rate  of  filtration  ;  this  varies  considerably  with 
different  brands  of  gelatin  and,  when  solutes  are 
present,  with  the  nature  of  the  latter. 

Gelatins  are  classified  as  "  hard  "  and  "  soft,"  the 
former  type  being  desirable  for  most  investigations. 
The  term  "  hardness  "  denotes  a  complex  of  qualities, 
among  which  are  high  "  melting  "  and  "  setting  " 
temperature  and  high  elastic  modulus.  The  melting 
and  setting  points  are,  of  course,  not  strictly  defined, 
and  can  be  determined  and  compared  only  by  con- 
ventional methods.  An  apparatus  suitable  for  this 
purpose  is  illustrated  in  Fig.  7.  A  test  tube  A  is 


46 


GELATIN  SOL. 


FIG.  7. 


suspended  in  the  centre  of  a 
30001400  c.c.  beaker  B,  which 
serves  as  a  water  bath,  by 
means  of  the  guide  C,  through 
which  it  must  slide  freely.  A 
tube  f"  diameter  x  6"  long  is 
suitable  ;  it  is  weighted  with 
15  to  20  gm.  of  mercury.  It 
is  essential  that  the  tube 
should  be  perpendicular  when 
it  is  resting  on  C  ;  if  the  rim 
is  not  sufficiently  regular  to 
ensure  this,  a  square  collar, 
say  of  rubber,  should  be  used 
and  permanently  attached  to 
the  tube.  A  glass  rod  D, 
about  f  diameter  for  a  |" 
tube,  is  suspended  exactly  in 
the  axis  of  the  test  tube.  (If 
the  apparatus  is  to  be  used 
frequently,  it  is  advisable  to 
mount  it  permanently  to 
ensure  correct  alignment.) 

To  determine  the  melting 
point  the  test  tube  is  rilled 
with  a  definite  quantity  of 
the  gelatin  sol  under  exami- 
nation, the  beaker  filled  with 
water  at  a  definite  tempera- 
ture, say  15°  C.,  and  the  sol 
allowed  to  set  for  a  definite 
time.  The  rod,  with  the  test 
tube  hanging  to  it,  is  now 
raised  a  definite  height  (which 
stage  is  shown  in  the  illus- 
tration), and  the  tempera- 
ture of  the  bath  slowly  raised, 
with  constant  stirring,  until 


MELTING  AND   SETTING  POINT.        47 

the  test  tube  slides  off  the  gel  cylinder  surround- 
ing the  rod  and  comes  to  rest  on  C.  The  tempera- 
ture at  this  moment  is  noted  as  the  "  melting 
point."  If  the  "  setting  point  "  is  also  to  be  deter- 
mined, the  rod  is  lowered  to  its  original  position,  the 
flame  extinguished,  and  the  bath  allowed  to  cool. 
The  rod  is  raised  very  slightly  from  time  to  time, 
until  it  just  lifts  the  test  tube  with  it,  the  tempera- 
ture at  this  point  being  noted  as  the  "  setting  point." 
It  must  be  remembered  that  there  is  considerable 
hysteresis  and  that  the  setting  point  of  harder  brands 
may  be  as  much  as  7°  or  8°  C.  lower  than  the  melting 
point  of  about  10  per  cent.  gels. 

A  more  delicate  method  of  determining,  with  very 
simple  means,  the  setting  point  is  based  on  the  well- 
known  fact  that  the  exposed  surface  of  a  gelatin  gel 
which  has  been  allowed  to  set  quietly  is  not  smooth 
like  that  of  a  liquid,  but  shows  a  network  of  wrinkles. 
The  formation  of  these  wrinkles  is  not  due  to  drying, 
but  occurs  actually  during  the  last  stage  of  setting. 
The  alteration  in  the  appearance  of  the  surface  is 
very  striking  if  it  is  observed  under  an  acute  angle  in 
reflected  light,  and  it  may  be  used  for  determining 
the  setting  point  in  the  following  manner :  A  small 
porcelain  crucible  is  filled  with  about  10  c.c.  of  sol 
and  the  bulb  of  the  thermometer  completely  immersed 
in  the  latter.  The  reflection  of  the  window  in  the 
surface  is  then  observed,  attention  being  fixed  on 
some  dark  object  in  the  light  field,  such  as  the  window- 
frame  or  the  like.  The  reflection  of  such  an  object 
is,  of  course,  distorted  by  the  menisci  formed  by  the 
sol  at  the  wall  of  the  crucible  and  the  stem  of  the 
thermometer,  but  is  a  smooth  and  unbroken  curve. 
As  soon  as  wrinkling  commences,  the  image  is  broken 
up  into  fringes  (see  Fig.  8,  a  and  b)  ;  the  fall  of 
temperature  between  the  time  when  this  altera- 
tion in  appearance  becomes  barely  perceptible  and 
when  it  is  quite  unmistakable  rarely  amounts  to 


48  GELATIN  GEL. 

more  than  0-1°,  which  is  a  more  than  sufficient 
accuracy. 

As  regards  other  physical  properties  of  the  sol,  the 
one  most  likely  to  require  investigation  is  its  vis- 
cosity at  different  concentrations  and  temperatures. 
The  methods  to  be  employed  are  described  fully  in 
the  chapter  dealing  with  viscosity  measurements. 
Since  viscosity  is  particularly  sensitive  to  variations 


B 


in  the  "  thermal  history,"  uniformity  of  procedure 
in  the  preparation  of  sols  for  this  purpose  must  once 
more  be  insisted  on  as  being  of  fundamental  import- 
ance. 

As  regards  the  gels,  reactions  in  gels  are  treated 
in  a  separate  chapter.  The  quantitative  study  of  the 
modulus  of  elasticity  or  the  accidental  birefringence 
produced  by  strain  is  beyond  the  limits  of  this  book. 
It  is,  however,  easy  to  demonstrate  the  latter  by  very 
simple  apparatus,  if  a  Nicoll  and  selenite  plate  are 


GELATIN  GEL. 


49 


available,  and  the  study,  particularly  of  the  strains 
set  up  by  drying,  is  instructive.  The  apparatus  is 
simply  an  open  box  (Fig.  9)  about  16"  high  X  8"  wide. 
Two  glass  plates — photographic  plates  from  which 
the  film  has  been  removed 
are  suitable  —  rest  on  the 
bottom,  inclined  under  an 
angle  of  about  53°  with  the 
latter.  A  strip  of  either 
ground-glass  or  glass  coated 
on  the  lower  side  with  the 
"  matt  varnish "  used  in 
photography  rests  on  two 
ledges,  about  9"  or  10"  from 
the  top  of  the  box.  An 
opening  in  the  centre  of  the 
top  takes  the  mount  of  the 
Nicoll  prism,  which  can  be 
rotated.  If  light  from  a  lamp 
placed  as  shown  is  reflected 
from  the  double  glass  plate, 
a  sufficient  fraction  of  it  is 
polarized  to  show  very  slight 
strains  in  gelatin  gels  con- 
taining 10  per  cent,  and 
more. 

The  strains  set  up  during 
drying  and  their  progressive 
changes  can  easily  be  traced 
and  are  instructive.  A  body 
of  gelatin  gel,  unless  it  is  a 
simple  surface  of  revolution 
approximating  fairly  closely 


FIG.  9. 


to  a  sphere,  does  not  remain  similar  to  itself  during 
drying,  and  if  the  surfaces  meet  in  edges  very  con- 
siderable distortion  occurs.  Thus,  a  right  cylinder 
with  flat  ends  has  two  circular  edges,  and  drying  is  at 
first  much  more  rapid  along  these  than  it  is  on  the 


50  AGAR  SOL. 

curved  or  flat  surfaces.  The  edges,  therefore,  con- 
tract and  the  cylinder  becomes  a  barrel  with  convex 
ends.  The  edges  have  now  become  so  dry  and  rigid 
that  very  little  further  drying  takes  place  in  them, 
while  the  rest  of  the  surface  is  rapidly  shrinking,  and 
the  final  shape  is  a  single-shell  hyperboloid  with  con- 
cave ends.  Similarly,  when  a  cube  is  allowed  to 
dry,  the  edges  contract  first  and  the  faces  become 
convex,  while  the  final  surface  has  concave  faces  with, 
of  course,  concave  edges.  The  distribution  of  strain, 
and  the  change  from  compression  to  tension,  can 
easily  be  observed  and  analyzed. 

Gelatin  being  highly  liable  to  putrefactive  changes, 
neither  sols  nor  gels  can  be  kept  for  long  without 
sterile  precautions,  which  are  beyond  the  scope  of 
this  work.  Hardening  agents  like  formaldehyde  alter 
the  physical  properties  of  gelatin  so  much  that  they 
are  suitable  only  for  preserving  finished  specimens. 
Directions  will  be  found  in  the  chapter  on  the 
Liesegang  phenomenon. 

A  gar  occurs  in  commerce  as  strips  having  a  fibrous 
texture,  as  a  fine  powder,  and  as  bars  of  square  cross- 
section.  The  first-named  is  the  cheapest  form ; 
powdered  agar  has  the  advantage  that  the  time 
necessary  for  swelling  is  considerably  reduced.  If 
strip  is  used  it  is  torn  into  small  pieces,  which  are 
allowed  to  swell  in  the  requisite  volume  of  water  for 
about  24  hours.  A  small  addition — one  part  in 
500 — of  acetic  acid  is  usual  and  promotes  imbibition, 
but  is  not  essential.  The  mixture  is  then  boiled 
slowly,  until  the  shreds  have  entirely  disappeared. 
The  sols  are  always  turbid  and  show  even  macro- 
scopic fibres  and  fragments,  so  that  they  must  at 
least  be  strained  through  fine  muslin  or  through 
glass  or  cotton-wool  plugs.  If  clearer  sols  and  gels 
are  required  they  must  be  filtered  through  Chardin 
paper  in  the  manner  described  for  gelatin ;  the  water 
in  the  jacket  must  be  boiling.  With  sols  containing 


AGAR  SOL.  51 

i  per  cent,  and  over  filtration  is  tediously  slow,  and 
the  filtrate  sets  long  before  the  filtration  of  even  small 
batches  is  complete.  If  a  large  drying  oven  or 
sterilizer  kept  at  100°  C.  is  available,  the  most  con- 
venient course  is  to  place  the  whole  apparatus,  i.e., 
filter  funnel  and  beaker  or  flask  for  the  filtrate,  into 
it,  when  the  whole  can  be  left  to  itself  without  further 
attention.  The  setting  temperature  of  agar  sols  is 
between  35°  and  40°,  while  the  melting  point  of  gels 
lies  between  90°  and  100°,  so  that  gels  have  to  be 
heated  on  a  water  bath  at  boiling  point  to  obtain  a 
sol.  Agar  gels  are  not  liable  to  putrefaction  and  are, 
therefore,  preferable  to  gelatin  for  long-continued 
experiments — e.g.,  on  diffusion — in  which  the  specific 
properties  of  the  gel  are  of  no  consequence.  They 
are,  however,  a  good  medium  for  the  growth  of  various 
moulds  and  occasionally  of  Bacillus  prodigiosus, 
which  latter  forms  red  patches.  Both  occur  chiefly 
on  the  surface,  and  the  rest  of  the  gel  may  generally 
be  used  after  the  affected  patches  have  been  cut 
away. 

Agar  gel,  unlike  gelatin,  does  not  adhere  to  glass, 
and  specimens  may  be  removed  from  moulds  with- 
out the  heating  necessary  in  the  case  of  the  latter. 
Thus  cylinders  may  be  cast  in  tubes  stoppered  at  the 
bottom,  and  will  drop  out  when  the  stopper  is 
removed.  A  certain  amount  of  liquid,  which  also 
contains  agar,  exudes  from  agar  gels  on  standing, 
partly  on  the  surface  and  partly  between  the  gel  and 
the  containing  vessel.  This  is  a  normal  phenomenon 
and  does  not  indicate  faulty  procedure  in  the  pre- 
paration. 

The  Lyotropic  Series. — It  is  desirable  to  demon- 
strate the  general  nature  of  the  series  by  showing  its 
effect  on  two  sols  as  chemically  different  as  gelatin 
and  agar.  Sulphates,  chlorides  and  thiocyanates 
may  be  chosen  as  representative  specimens,  suffi- 
cient of  each  being  placed  into  a  loo-c.c.  beaker  to 

4—2 


52  THE  LYOTROPIC  SERIES. 

produce  a  concentration  of  N/2  in  50  c.c.  of  sol  (allow 
for  water  of  crystallization !) .  Each  of  the  beakers 
so  prepared  now  receives  50  c.c.  of  sol,  10  per  cent, 
gelatin  sol  and  i  per  cent,  agar  sol  being  suitable, 
and  the  same  quantity  of  pure  sol  is  placed  into  a 
fourth  beaker  for  comparison.  All  four  beakers  are 
placed  in  the  water  bath  until  they  have  attained 
the  same  temperature  and  are  then  taken  out  and 
allowed  to  cool.  The  water  bath  should  be  at  35° 
to  40°  C.  for  the  gelatin  sol  and  at  boiling  point  for  the 
agar  sol.  The  order  in  which  the  gels  set  will  be  the 
same  for  gelatin  and  agar,  and  the  intervals  between 
the  four  specimens  will  be  considerable  ;  the  sol 
containing  thiocyanate  remains  liquid  at  room  tem- 
perature. 

The  effect  of  the  lyotropic  series,  and  also  that  of 
dilute  acid  and  alkali,  on  the  swelling  of  gelatin 
may  also  be  demonstrated  very  simply  in  the  follow- 
ing manner.  Squares  having  a  side  of,  say,  15  mm. 
are  cut  from  the  gelatin  foil  mentioned  above  ;  if 
this  is  not  obtainable  leaf  may  be  used,  but  the 
diamond  markings,  which  are  highly  strained,  should 
be  avoided.  The  squares  are  placed  in  watch-glasses 
or  Petri  dishes  containing  a  few  cubic  centimetres  of 
the  following  solutions  :  N/50  HC1,  N/5O  NaOH, 
N/i  Na2SO4,  N/2  NaCl,  N/2  NH4CNS  and  water. 
The  squares  should  be  held  in  a  small  forceps  and 
immersed  quickly  and  completely,  without  allowing 
air  bubbles  to  adhere  to  them.  The  difference  in 
swelling  will  be  quite  noticeable  after  one  hour  (when 
the  gelatin  placed  in  the  thiocyanate  solution  is 
probably  completely  dispersed),  although  complete 
equilibrium  is  not  attained  for  many  hours.  It  must 
be  remembered  that  the  foil  swells  in  all  directions 
and  that  the  increase  in  volume  is,  therefore,  pro- 
portional to  the  cube  of  the  side.  The  squares  are 
best  examined  by  holding  the  watch-glass  2"  or  3" 
above  a  black  background,  when  they  appear 


PURIFIED  GELATIN.  53 

turbid,  or  of  course  by  carefully  pouring  off  the 
liquid. 

Purified  Gelatin. — The  work  described  so  far  can 
be  carried  out  with  the  raw  material  obtainable  com- 
mercially. This  always  contains  electrolytes,  which 
it  may  be  necessary  to  remove  as  far  as  possible, 
although  it  should  be  noted  that  the  physical  pro- 
perties of  the  gelatin  are  sensibly  affected  by  the 
prolonged  washing  which  is  required.  A  known 
quantity  of  the  leaf  is  placed  in  a  weighed  tall  beaker, 
capable  of  holding  a  volume  of  water  equal  to  at 
least  15  times  the  weight  of  gelatin.  Running  water 
is  then  passed  in  near  the  bottom  of  the  vessel  and 
allowed  to  overflow  for  48  hours.  If  an  adequate 
supply  of  distilled  water  is  available  it  may  be  used  ; 
failing  this,  tap -water  may  be  employed,  in  which 
case  washing  must  be  completed  with  several  changes 
of  distilled  water.  To  prevent  the  formation  of 
mould  a  few  fragments  of  camphor  or  thymol, 
wrapped  in  muslin,  are  added  between  the  leaves, 
so  as  not  to  escape  with  the  water. 

Since  gelatin  imbibes  something  like  10  times  its 
weight  of  water,  sols  of  greater  concentration  cannot 
be  made  directly  from  the  washed  gelatin  by  warming 
to  dispersion.  To  obtain  them  it  is  necessary  either 
to  dry  the  wet  mass  over  H2S04  or  CaCl2,  or  to 
evaporate  the  dilute  sol  to  the  required  concentra- 
tion at  fairly  low  temperature,  the  concentration  in 
either  case  being  determined  by  weighing.  The 
second  procedure  affects  the  sol  perceptibly,  especially 
if  prolonged. 

Sols  containing  a  Definite  Amount  per  Volume. — 
We  have  so  far  dealt  only  with  sols  and  gels  contain- 
ing a  definite  percentage  of  gelatin  to  a  given  amount 
of  water;  in  other  words,  with  sols  containing  a 
definite  amount  of  gelatin  in  a  given  weight  of  sol. 
The  preparation  of  sols  containing  a  definite  amount 
of  substance  in  a  given  volume  is  complicated  chiefly 


54  GELATIN  CONCENTRATION 

by  the  fact  that  sols  are  not  liquid  at  the  tempera- 
tures for  which  the  usual  measuring  vessels  are 
graduated,  and  the  first  point  to  decide  is  whether 
the  sol  is  required  to  have  a  definite  concentration 
at  some  particular  temperature,  say  35°  for  vis- 
cosity measurements,  or  at  some  arbitrarily  chosen 
lower  temperature,  at  which  it  may  be  transformed 
into  gel.  In  the  former  case  the  flask  to  be  used 
should  be  filled  with  water  at  the  standard  tempera- 
ture and  then  placed  in  a  water  bath  and  warmed  to 
the  temperature  selected  ;  a  fresh  mark  should  be 
placed  at  the  level  reached  by  the  water.  The  volume 
to  this  mark  is  calculated  from  the  ratio  of  the 
specific  volumes  of  water  at  the  two  temperatures 
selected ;  for  15°  and  35°  C.  respectively  these  are,  for 
instance,  1-00085  and  1-00586,  so  that  the  volume 
of,  say,  500  c.c.  measured  at  15°  will  be 

500  x  1-00586  =  502-5  c.c.  at  35°, 

1-00085 

which  figure  is  noted  and  the  gelatin  content  cal- 
culated on  it.  Since  it  is  inconvenient  to  soak  and 
disperse  the  leaf  in  a  long-necked  flask,  this  should 
be  done  in  a  beaker  with  about  75  or  80  per  cent,  of 
the  total  volume  of  water  required,  the  more  con- 
centrated sol  thus  obtained  poured  into  the  flask, 
which  is  placed  in  a  thermostat  at  the  required  tem- 
perature, and  the  beaker  washed  out  with  successive 
small  portions  of  warm  water,  which  are  transferred 
to  the  flask  until  the  mark  is  reached.  The  contents 
of  the  flask,  of  course,  require  thorough  mixing 
before  use. 

Commercial  leaf  contains  a  considerable  amount  of 
moisture,  rarely  less  than  10  per  cent.,  which  must 
be  taken  into  account.  It  can  be  removed  almost 
entirely  by  drying  at  100°  to  constant  weight  (note 
that  gelatin  takes  up  moisture  from  the  air  even 
during  the  time  required  for  weighing),  which  treat- 
ment, however,  affects  the  properties  of  the  material 


IN  A  GIVEN  VOLUME.  55 

very  considerably.  It  should,  therefore,  be  applied 
only  to  a  small  sample,  the  rest  of  the  material  being 
kept  in  an  airtight  receptacle  from  the  time  at  which 
the  sample  has  been  taken,  so  that  its  moisture  con- 
tent remains  constant.  When  making  up  sols  to  a 
given  concentration  per  volume,  this  should,  of 
course,  be  calculated  on  the  weight  of  dry  gelatin. 

It  should  be  remembered  that  sols  made  by 
diluting  a  more  concentrated  sol  differ  slightly  in 
their  physical  properties  from  sols  produced  by  dis- 
persing the  gelatin  at  once  in  the  total  water  required. 
To  obtain  comparable  results  it  is  again  necessary 
to  observe  the  rigid  uniformity  of  procedure  which 
has  already  been  insisted  on,  i.e.,  to  use  the  same 
percentage  for  diluting,  to  have  the  added  water  at 
the  same  temperature  as  the  sol,  etc. 

If  powdered  gelatin  (the  usual  brands  of  which  are, 
however,  markedly  less  "  hard "  than  the  best 
brands  of  leaf)  can  be  used,  the  procedure  is  simpler, 
since  there  is  no  difficulty  in  carrying  out  soaking 
and  dispersion  in  the  measuring  flask  itself.  The 
latter  should  be  about  half  filled  with  water  by  means 
of  a  long  (thistle)  funnel,  so  that  the  neck  remains 
quite  dry.  The  powdered  gelatin  is  then  poured  in, 
in  small  portions  and  in  a  thin  stream,  through  a  wide- 
necked  funnel,  and  the  flask  shaken  frequently  to 
cause  the  powder  to  sink  without  the  formation  of 
lumps.  On  no  account  must  the  powder  be  placed 
in  the  dry  flask  first.  When  the  whole  of  the  powder 
is  submerged  further  water  is  added  to  within  about 
5  c.c.  of  the  mark  giving  the  volume  at  15°  C.  (this 
refers  to  a  5oo-c.c.  flask  ;  for  other  sizes  the  margin 
should  be  in  proportion),  and  the  necessary  time 
allowed  for  swelling.  The  flask  is  then  placed  in  the 
water  bath,  and  the  volume  is  made  up  to  the  mark 
giving  the  volume  at  the  working  temperature  which 
has  been  fixed  upon.  Careful  mixing  before  use  is 
also  required  in  this  case,  since  the  gelatin  does  not 


56  GELATIN  SOL. 

diffuse  perceptibly  during  dispersion,  so  that  a  con- 
centrated layer  of  it  rests  on  the  bottom. 

LITERATURE. 

A  very  complete  Study  of  "Vapour  Pressure,  etc.,  of 
Gelatin- Water  Systems,"  by  K.  Gericke,  Kott.-Zeitschr., 
XVII.,  78  (1915)  ;  "Influence  of  Neutral  Salts  on  Vis- 
cosity and  Swelling,"  J.  Loeb,  Journ.  of  BioL  Chem.,  34, 
77,  345 


CHAPTER  VII. 
EGG  ALBUMIN  SOL. 

THE  only  commercial  raw  material  is  dried  egg 
albumin,  and  the  beginner  should  carry  out  the 
experiments  described  below  with  a  sol  made  from 
it ;  although  its  use  is  open  to  objections,  the  results 
obtainable  correspond  sufficiently  closely  to  those 
recorded  in  the  literature  for  "  natural  "  albumin. 

Crush  15  gm.  of  dried  egg  albumin  coarsely  and 
introduce  in  small  portions,  with  stirring,  into 
100  c.c.  of  water.  The  albumin  at  first  adheres  to 
the  walls  of  the  vessel  with  great  tenacity,  but  is 
easily  detached  as  imbibition  proceeds.  The  sol 
should  be  stirred  from  time  to  time  and  lumps 
broken  up  until  dispersion  is  complete.  The  sol  is 
turbid,  with  a  varying  small  fraction  of  insoluble 
matter,  which  does  not  settle  even  on  prolonged 
standing.  It  must  be  filtered,  preferably  overnight, 
through  asbestos  in  the  manner  described  on  p.  12  ; 
filtration  through  paper  is  extremely  tedious  and 
involves  considerable  loss.  The  filtrate  from  asbes- 
tos is  a  yellowish  liquid,  opalescent,  but  quite  suffi- 
ciently clear  in  moderate  thickness,  say  in  test  tubes 
15  or  1 8  mm.  diameter,  to  allow  even  incipient  pre- 
cipitation to  be  noticed  easily.  Albumin  sols  are 
very  liable  to  undergo  decomposition  and  should  be 
used  quite  fresh  ;  in  warm  weather  a  trace  of  thymol 
may  be  added  to  the  mixture  before  filtration  without 
affecting  the  properties  of  the  sol. 

Heat  Coagulation. — Place  a  test  tube  containing 
about  10  c.c.  of  sol  in  a  small  water  bath  and  heat 
slowly,  stirring  constantly  with  a  thermometer. 
Note  the  temperature  at  which  the  sol  begins  to  turn 


58  EGG  ALBUMIN  SOL. 

white  and  opaque.  Remove  the  test  tube  from  the 
bath,  add  water  and  break  up  the  coagulum,  to  show 
that  it  does  not  disperse  again,  i.e.,  that  the  heat 
coagulation  is  irreversible. 

Irreversible  Change  by  Adsorption. — Albumin  is 
readily  adsorbed  at  the  interface  between  sol  and 
another  liquid,  and  becomes  insoluble  in  the  process. 
This  is  easily  demonstrated  by  placing  in  a  test  tube 
10  c.c.  of  sol,  adding  i  c.c.  of  some  organic  liquid 
heavier  than  the  sol,  e.g.,  chloroform  or  carbon 
tetrachloride,  and  shaking  vigorously,  so  that  added 
liquid  is  broken  up  into  small  drops.  These  sink  to 
the  bottom  and  remain  perfectly  separate,  which 
shows  the  formation  of  a  film  preventing  their 
coalescence  and  evidently  insoluble  in  the  sol,  as  no 
change  takes  place  even  on  standing  for  some  time. 
To  demonstrate  that  the  adsorbed  film  is  also 
insoluble  in  water,  pour  off  the  sol  and  replace  by 
water  ;  no  coalescence  occurs  even  then. 

Salting-out  and  the  Hofmeister  Series  of  Anions. — 
It  will  be  sufficient  to  try  a  few  of  the  more  charac- 
teristic salts  of  the  series  by  adding  the  dry  salts  to 
the  same  volume  of  sol,  so  as  to  keep  the  albumin 
concentration  approximately  constant.  The  molar 
concentrations  necessary  to  produce  immediate  tur- 
bidity in  "  natural  "  albumin  sols  (i.e.,  sols  contain- 
ing, like  ours,  the  other  constituents  of  egg-white) 
are  given  below,  as  well  as  the  amounts  of  the  most 
readily  obtainable  salts  required  to  produce  these 
concentrations  in  10  c.c.  volume : — 

M.  per  litre.  Gm.  in  10  cc.  of  solution. 

Na  citrate  .  0-56  1/647  crystallized  neutral  sodium 
citrate. 

Na2S04  .  0-80  2-577  crystallized  sodium  sul- 
phate. 

NaCH3.C02  1-69  2*298  crystallized  sodium  acetate. 

NH4CNS  .  —  Ammonium  thiocyanate  to  satu- 
ration. 


EGG  ALBUMIN  SOL.  59 

Place  the  coarsely  powdered  salts  in  flasks  with  a 
mark  at  10  c.c.,  or,  if  these  are  not  available,  in  test 
tubes  15  or  18  mm.  diameter  which  have  been  pro- 
vided with  a  mark  at  that  volume.  Place  the  flasks 
or  test  tubes  in  a  water  bath  kept  at  about  35°  and 
dissolve  the  salts  gradually  by  slowly  reversing  the 
tubes  at  intervals  ;  they  must  not  be  shaken,  as  the 
formation  of  froth  is  to  be  avoided.  Watch  the 
appearance  as  solution  proceeds  and  note  that  a 
marked  turbidity  appears  only  when  the  whole  of 
the  salt  has  gone  into  solution,  except  with  thio- 
cyanate,  which  does  not  salt  out  even  in  saturated 
solution. 

Dilute  the  turbid  sols  with  an  equal  volume  of 
water,  and  note  that  they  become  clear,  i.e.,  the 
salting  out  is  reversible. 

Reversal  of  the  Hofmeister  Series  in  Acid  Sols. — 
Acidify  20  c.c.  of  sol  by  adding  i  c.c.  of  normal 
hydrochloric  acid.  Place  in  one  test  tube  the  same 
quantity  of  sodium  sulphate  (2-577  gm-)  as  use(i 
above,  and -in  another  3  gm.  of  ammonium  thio- 
cyanate,  fill  to  the  10  c.c.  mark  with  the  acid  sol,  and 
dissolve  the  salts  gradually.  No  precipitate  is  formed 
in  the  sol  containing  the  sulphate,  while  the  sol  con- 
taining thiocyanate  becomes  turbid  and  eventually 
clots  completely. 

Precipitation  by  Salts  of  the  Heavy  Metals. — Pre- 
pare some  2N  solution  of  copper  sulphate  (say 
12-480  gm.  of  the  crystallized  salt  in  50  c.c.  of 
solution).  Add  from  a  burette  a  few  drops  of  this 
solution  to  10  c.c.  of  sol ;  a  heavy  greenish  coagulum 
forms  immediately.  Continue  to  add  copper  sul- 
phate solution,  with  occasional  stirring ;  the  pre- 
cipitate re-dissolves  and  has  disappeared  when  10  c.c. 
of  solution  has  been  added,  i.e.,  when  the  mixture  is 
normal  in  respect  of  copper  sulphate.  Now  add 
sufficient  powdered  copper  sulphate  to  saturate  the 
20  c.c.  (about  5-6  gm.)  and  dissolve ;  a  second 


60  PURIFICATION  OF 

precipitation  begins  when  the  solution  has  become 
saturated. 

To  show  that  coagulation  by  salts  of  heavy  metals 
is  irreversible,  repeat  the  experiment  as  far  as  adding 
a  few  drops  of  copper  sulphate  solution  to  10  c.c.  of 
sol,  then  dilute  with  water,  and  note  that  the  coagu- 
lum  does  not  dissolve. 

The  student  desirous  of  working  with  pure  albumin 
will  do  well  to  practise  the  usual  methods  of  purifica- 
tion in  the  first  instance  with  dried  albumin.  The 
methods  are  based  on  the  fact  that  the  constituents 
of  white  of  egg  other  than  albumin,  viz.,  globulin, 
ovomucoid,  etc.,  are  salted  out  by  lower  concen- 
trations of  ammonium  sulphate  than  is  albumin. 
Disperse  15  gm.  of  dried  egg  albumin  in  100  c.c.  of 
water,  as  described,  but  do  not  filter  the  sol.  Add 
in  small  portions  sufficient  finely  powdered  pure 
ammonium  sulphate  to  produce  a  half-saturated 
solution  ;  38  to  39  gm.  is  required.  Each  addition 
should  be  made  only  after  the  previous  one  has  dis- 
solved. A  white  coagulum  forms  and  is  removed  by 
filtration  through  a  folded  paper  filter.  The  filtrate 
contains  the  albumin  dispersed  in  half-saturated 
ammonium  sulphate  solution.  The  salt  can  now  be 
removed  by  dialysis  in  parchment,  or,  better,  collo- 
dion against  running  water,  and  a  moderately  pure 
albumin  sol  obtained.  The  usual  method  is,  how- 
ever, to  precipitate  the  albumin  by  saturating  the 
solution  with  ammonium  sulphate,  a  further  38  to 
39  gm.  being  required  for  every  100  c.c.  of  filtrate. 
A  thick  coagulum  of  albumin  forms,  which  is  filtered 
overnight  and  allowed  to  drain  as  far  as  possible. 
The  residue,  which  always  contains  a  considerable 
amount  of  mother  liquor,  is  then  dissolved  in  the 
smallest  volume  of  water  which  will  give  a  clear  sol, 
and  dialysed  as  explained  above  to  remove  ammonium 
sulphate. 

The  same  method  is  applied  to  fresh  white  of  eggs. 


EGG  ALBUMIN  SOL.  61 

About  28  to  30  c.c.  of  white  can  be  obtained  from 
average  fowls'  eggs  ;  this  contains  about  10  per  cent, 
of  albumin.  The  total  protein  content  is  about 
12-2  per  cent.,  the  difference  being  accounted  for  by 
globulin  and  mucoid.  The  egg-white  is  beaten  up 
with  an  equal  volume  of  saturated  ammonium  sul- 
phate solution,  which  produces  half -saturation  in 
the  mixture  and  precipitates  the  latter  constituents. 
The  coagulum  is  filtered  off  and  the  filtrate  saturated 
with  ammonium  sulphate  to  precipitate  the  albumin, 
which  is  filtered  off  and  dissolved  in  a  small  volume 
of  water.  This  sol  is  again  precipitated  by  satura- 
tion with  ammonium  sulphate,  and  the  previous 
operations  repeated  ;  several  re -precipitations  are 
required  to  obtain  pure  albumin.  The  last  coagulum 
is  dissolved  in  a  small  volume  of  water  and  dialysed 
to  remove  the  sulphate.  The  losses  are  fairly  con- 
siderable, and  the  albumin  content  of  the  sol  finally 
obtained  after  dialysis  will  be  between  2  and  2-5  gm. 
of  albumin  for  every  30  c.c.  of  white  used  originally. 
Although  the  general  student  will  hardly  have 
occasion  to  use  it,  the  classical  method  of  making 
"  crystallized  albumin  "  is  here  given.  It  may  be 
tried  with  the  white  of  two  or  three  eggs,  which 
should  be  perfectly  fresh.  The  volume  of  egg-white 
is  measured  and  an  exactly  equal  volume  of  saturated 
ammonium  sulphate  solution  added  to  it  in  small 
portions  at  a  time,  the  mixture  being  vigorously 
beaten  with  an  egg-beater  after  each  addition  until 
the  whole  has  been  reduced  to  a  stiff  froth.  This  is 
allowed  to  stand  overnight,  and  is  then  filtered  to 
remove  the  coagulum  of  globulin,  etc.  Ten  per  cent, 
acetic  acid,  i.e.,  glacial  acetic  acid  diluted  to  10 
times  its  volume,  is  then  added  to  the  nitrate  from 
a  burette,  a  single  drop  at  a  time,  with  gentle  stirring 
to  re-dissolve  the  precipitate  formed  locally  before  a 
further  drop  is  added.  This  is  continued  until  the 
solution  becomes  permanently  -turbid — the  exact 


62         CRYSTALLIZED  EGG  ALBUMIN 

degree  of  turbidity  can  only  be  found  by  practice, 
but  must  amount  to  something  more  than  mere 
opalescence.  When  this  point  has  been  reached, 
i  c.c.  of  acid  for  every  100  c.c  of  solution  is  added.  A 
copious  precipitate  forms,  which,  on  standing  and 
occasional  gentle  shaking,  becomes  (micro-)  crystal- 
line after  five  or  six  hours  ;  to  obtain  the  full  yield 
it  should,  however,  be  allowed  to  stand  for  24  hours. 
The  precipitate  is  filtered  off  and  dissolved  in  a  small 
volume  of  water  ;  the  solution  is  then  dialysed,  or, 
if  further  purification  is  desired,  it  is  again  preci- 
pitated. This  is  done  by  dissolving  the  coagulum 
from  the  filter  in  the  smallest  possible  volume  of 
water,  acidifying  with  a  few  drops  of  10  per  cent, 
acetic  acid,  and  then  adding  concentrated  ammonium 
sulphate  until  a  slight  permanent  turbidity  results. 
After  24  hours'  standing  the  bulk  of  albumin  has  been 
re-precipitated. 

The  beginner  will  find  the  exact  degree  of  turbidity 
required  somewhat  difficult  to  judge  and  must  be 
prepared  for  disappointment.  In  dialysing  albumin 
sols  remember  what  has  been  said  on  page  27  regard- 
ing sols  which  exert  an  appreciable  osmotic  pressure 
and  use  suitable  arrangements. 

LITERATURE. 

This  is  too  voluminous  to  allow  of  being  summarised. 
Students  must  consult  the  text-books  on  Proteins  or 
those  of  Biochemistry. 


CHAPTER  VIII. 

EMULSIONS. 

BOTH  as  regards  methods  of  preparation  and  pro- 
perties these  fall  into  two  classes,  which  are  best 
studied  separately  :  the  pure  oil-water  emulsions,  in 
which  no  solute  is  present  in  the  water,  and  the  con- 
centrated emulsions,  which  can  be  produced  only  by 
adding  to  the  water  phase  certain  substances  which 
greatly  lower  its  surface  tension  and  occasionally 
possess  other  properties  as  well. 

Pure  Oil-Water  Emulsions. — These  are  most  con- 
veniently prepared  by  the  following  method  :  o-i  c.c. 
of  the  oil  (which  may  be  a  paraffin  oil  of  low  vis- 
cosity, oleic  acid,  or  generally  any  other  liquid 
immiscible  with  water,  but  soluble  in  alcohol)  is 
dissolved  in  10  c.c.  of  alcohol  or  acetone.  This 
solution  is  blown  from  a  pipette  into  one  litre  of 
water  ;  the  water  is  well  agitated  before  immersing 
the  pipette,  the  point  of  which  should  be  10  to  15  cm. 
below  the  surface.  The  resulting  emulsion  should 
show  a  bluish  tinge  in  reflected  light  (particularly 
well  marked  with  oleic  acid),  and  be  practically  clear 
in  transmitted  light. 

The  emulsion  should  be  examined  with  a  dark- 
ground  condenser  and  the  sign  of  the  charge  deter- 
mined in  the  cataphoresis  apparatus.  The  coagula- 
tion by  HC1  should  be  watched  under  the  microscope 
as  the  phenomenon,  viz.,  coalescence  of  discharged 
particles  to  bigger  ones,  with  decreasing  amplitude 
of  Brownian  movement,  is  slow  and  more  easily 
followed  than  with  suspensoids,  with  the  behaviour 
of  which  it  otherwise  agrees. 


64  EMULSIONS. 

Electrolyte  coagulation  should  be  tried  with  HC1 
and  with,  say,  CaCl2  and  A12(S04)S ;  salts  of  univa- 
lent  cations  act  only  in  very  great  concentrations. 
The  effect  of  the  coagulant  shows  itself  macro- 
scopically  by  the  disappearance  of  the  bluish  opales- 
cence,  the  emulsion  becoming  whitish  and  turbid 
instead.  Samples  should  be  taken  at  intervals  and 
examined  microscopically  (ordinary  illumination, 
using  sub-stage  condenser  and  a  fairly  small 
diaphragm,  magnification  about  600  diameters), 
when  it  will  be  found  that  globules  about  3/1,  diameter 
gradually  take  the  place  of  smaller  ones,  this  being 
the  size  at  which  Brownian  movement  becomes  so 
sluggish  that  further  collisions  between  globules, 
and  therefore  formation  of  larger  ones.,  practically 
cease  to  occur. 

The  emulsions  to  which  sufficient  coagulant  has 
been  added  gradually  clear  from  the  bottom  upwards, 
provided  the  "  oil  "  has  a  density  lower  than  that  of 
water.  The  rate  of  clearing  should  be  measured  at 
convenient  intervals,  24  or  48  hours,  according  to  the 
difference  in  density,  and  the  size  of  the  globules 
calculated  from  Stokes's  formula  (determine  density 
of  oil  to  three  decimals) . 

Concentrated  Emulsions. — To  prepare  these  it  is 
necessary  to  lower  the  surface  tension  of  the  aqueous 
phase,  the  most  convenient  agent  for  the  purpose 
being  a  soap.  Either  a  soap  solution  may  be  used, 
the  preparation  of  which  will  be  described  below,  or 
the  soap  may  be  actually  produced  in  the  process  of 
emulsification.  This  method,  which  of  course  is 
applicable  only  to  oils  which  are  glycerides,  consists 
in  shaking  up  the  oil  with  a  dilute  solution  of  caustic 
soda,  N/50  to  N/ioo  being  suitable  concentrations. 
Small  quantities  may  be  prepared  in  test  tubes ; 
pour  10  c.c.  of  the  NaOH  solution  into  a  test  tube  of 
25  to  30  c.c.  capacity,  then  add  ordinary  olive  or 
cotton-seed  oil  in  lots  of  i  c.c.,  close  the  cest  tube 


RATIO  OF  PHASES.  65 

with  the  thumb  and  shake  vigorously  after  each 
addition.  The  emulsion  becomes  a  pure  white 
(why  ?},  and  after  the  addition  of  about  10  c.c.  of  oil 
the  viscosity  increases  so  much  that  the  dispersion  of 
further  oil  becomes  difficult.  Larger  quantities  may 
be  prepared  in  the  same  way  in  any  shaking  apparatus 
which  may  be  available ;  in  this  case,  too,  the  oil 
should  be  gradually  added  in  small  portions. 

The  oil  in  emulsions  thus  prepared  gradually  rises, 
a  sharp  boundary  forming  between  the  concentrated 
emulsion  at  the  top  and  the  dispersion  medium, 
which  is  turbid  owing  to  the  presence  of  soap  and 
very  fine  particles.  This  rise  continues  until  the  oil 
globules  are  in  closest  packing ;  as  they  are  not  of 
uniform  size,  no  exact  figure  can  be  given  for  the 
percentage  of  disperse  phase,  but  it  will  be  found 
to  be  70  per  cent,  or  over.  The  volume  ratio  can 
be  determined,  with  small  errors  due  to  contrac- 
tion, etc.,  in  the  following  way  :  a  burette  is  filled  to 
the  lowest  mark  (i.e.,  the  one  bearing  the  highest 
number)  with  dilute  hydrochloric  acid.  The  emul- 
sion is  then  poured  into  the  burette,  the  volume 
noted,  the  burette  closed  with  the  thumb,  and  emul- 
sion and  acid  thoroughly  mixed.  The  oil  separates 
and  rises ;  any  small  globules  which  may  remain  sepa  - 
rated  from  the  main  bulk  must  be  made  to  unite 
with  it  by  tapping  and  inclining  the  burette.  The 
volume  of  oil  is  then  read  off  and  the  volume  of 
continuous  phase  obtained  by  difference. 

When  mineral  oils  are  to  be  emulsified  the  proce- 
dure described  is  not  applicable,  but  soap  solution 
must  be  employed.  Ammonium  oleate  is  extremely 
efficacious,  but  is  not  obtainable  commercially,  and 
sodium  oleate  (olive  oil  or  Marseilles  soap)  will 
generally  have  to  be  used.  It  is  cut  into  fine 
shavings,  which  are  allowed  to  dry  in  air  for  three  or 
four  days,  and  10  gm.  of  the  air-dried  material 
dissolved  in  one  litre  of  distilled  water,  at  30°  to 


66 


Jl 


FIG.  10. 


SIMPLE  APPARATUS  FOR 


40°  C.  The  solution  is  allowed  to 
stand  in  the  cold  for  24  hours  and 
then  filtered  twice  through  the  same 
filter  of  fairly  open  paper. 

Emulsiiication  may  be  accom- 
plished by  shaking,  as  described 
above.  If  no  shaking  device  is 
available,  fair  quantities  may  be 
prepared  in  the  apparatus  shown  in 
Fig.  10,  which  may  be  made  up 
from  vessels  to  be  found  in  the 
laboratory.  A  tall  cylinder  is 
closed  by  a  rubber  stopper  with 
two  perforations.  A  thistle  funnel, 
having  a  tube  3  to  4  mm.  diameter, 
reaching  to  within  3  or  4  mm.  of 
the  bottom  of  the  cylinder,  passes 
through  one  of  the  perforations ;  a 
ball  tube,  with  two  or  three  balls, 
through  the  other.  A  large  pipette 
— 50  to  100  c.c. — with  its  lower 
end  drawn  to  a  capillary  point,  is 
suspended  above  the  thistle  funnel 
so  that  the  point  touches  the  wall 
of  the  funnel.  The  point  is  made 
so  fine  that  50  c.c.  of  oil  of  low 
viscosity,  like  the  paraffins  used  in 
lamps,  takes  25  to  30  minutes  to 
empty.  If  the  flow  is  found  too 
rapid  it  may  be  reduced  by  fitting 
a  short  length  of  rubber  tubing, 
provided  with  a  screw  clip,  to  the 
upper  end  of  the  pipette. 

The  apparatus  is  used  as  follows : 
The  stopper  is  removed  and  a 
known  volume  of  soap  solution 
poured  into  the  cylinder ;  it  should 
not  exceed  one- third  of  the  total 


PREPARING  EMULSIONS.  67 

volume.  A  few  drops  of  the  oil  to  be  emulsified 
are  then  poured  down  the  funnel  and  the  latter 
rotated  slowly,  so  that  the  whole  of  the  tube  is 
wetted  by  oil.  The  stopper  is  then  replaced,  \he 
pipette  filled  with  oil  suspended  as  explained  above, 
and  the  ball  tube  connected  to  the  filter  pump. 
The  latter  should  be  so  adjusted  that  the  air  issues 
in  a  uniform  string  of  separate  bubbles  at  the 
bottom  of  the  funnel  tube.  When  the  pump  is 
working  properly,  the  clip  at  the  top  of  the  pipette  is 
opened  to  the  required  extent,  and  the  apparatus 
then  requires  no  further  attention.  The  oil,  which 
runs  down  the  tube  in  a  very  thin  film,  is  broken  up 
by  the  air  bubbles  when  passing  out  at  the  lower 
edge  of  the  tube,  and  thorough  emulsification  takes 
place.  Frothing  is  rather  marked  at  the  beginning, 
but  subsides  after  a  little  oil  has  been  emulsified,  and 
the  ball  tube  prevents  froth  from  being  drawn  into 
the  suction  tube  to  any  extent. 

The  emulsions  made  with  soap  solution  separate  a 
"  cream,"  like  those  prepared  with  caustic  soda,  and 
the  same  methods  may  be  used  for  determining  the 
volume  ratio. 

Emulsions  which  do  not  separate,  whatever  the 
volume  ratio,  can  be  made  from  oils  which  have  the 
same  density  as  the  dispersion  medium.  The 
simplest  way  is  to  prepare  suitable  mixtures  of  either 
olive  oil,  cotton-seed  oil,  or  lamp  paraffin  with 
carbon  tetrachloride  (density  at  o°  1*632).  The 
ratio  may  be  approximately  calculated  from  this 
and  the  density  of  the  oil ;  as,  however,  the  co-effi- 
cients of  expansion  of  the  aqueous  dispersion  medium 
and  the  mixture  of  oil  and  CC14  differ  widely,  exact 
equality  can  only  be  secured  by  experiment  at  a 
definite  temperature.  For  this  purpose  the — approxi- 
mate— mixture  and  a  small  beaker  filled  with  the 
soap  solution  are  placed  in  the  thermostat,  a  i  c.c. 
pipette  filled  with  the  former,  and  slowly  blown  out 

5-2 


68  EMULSIONS. 

under  1he  surface  of  the  soap  solution,  so  that  a 
single  drop  is  formed,  which  can  easily  be  detached 
from  the  pipette.  According  as  this  drop  rises  or 
sinks,  more  CC14  or  more  oil  is  added  to  the  mixture, 
until  the  drop  remains  practically  stationary  for  a 
few  minutes.  Emulsions  of  this  kind  are  especially 
suitable  for  viscosity  measurements  (which  see). 

LITERATURE. 

For  recent  papers,  see  article  "  Emulsions  "  in  Second 
Report  of  British  Association  Committee  on  Colloid 
Chemistry,  1918,  p.  20. 


CHAPTER  IX. 
ULTRA-FILTRATION. 

THE  name  was  given  by  H.  Bechhold  to  a  method 
of  separating  the  disperse  phase  of  sols  from  the 
dispersion  medium  by  means  of  nitration  under 
pressure  through  porous  membranes  impregnated 
with  gels,  the  permeability  of  which  may  be  varied 
within  wide  limits.  As  considerable  pressures  may 
have  to  be  used  metal  apparatus  is  essential,  which 
limits  the  applicability  of  the  method  to  some 
extent.  An  apparatus  suitable  for  sols  containing 
a  very  small  amount  of  solid  only  (as  is  the  case  with 
most  suspensoid  sols)  is  illustrated  in  Fig.  u.  The 
filtering  membrane  rests  on  a  perforated  metal  disc  a, 
which  is  clamped  between  the  body  b  of  the  filter 
and  the  slightly  conical  bottom  c  and  rests  on  six 
radial  ribs  in  the  latter.  The  branch  d,  closed  by  a 
screw  cap,  serves  for  filling  the  filter,  and  the 
necessary  pressure  is  generated  by  a  bicycle  tyre 
pump  connected  to  the  valve  e.  The  joint  between 
the  vessel  and  the  cover  is  made  tight  by  a  rubber 
i  ing  cemented  to  the  spigot  on  the  former  with  marine 
glue  or  Chatterton's  compound.  To  prepare  the 
filter  for  use,  the  cover  with  the  perforated  metal 
plate  is  removed,  the  membrane  placed  carefully  on 
the  latter  ;  the  cover  is  then  replaced  and  the  two 
nuts  tightened.  The  filter  is  then  charged  through 
the  large  inlet,  the  cap  replaced  and  tightened,  and 
pressure  generated  by  means  of  the  bicycle  pump. 
The  filter  should  not  be  more  than  about  half  full, 
so  as  to  leave  a  sufficient  air  space,  as  otherwise  the 


70  ULTRA-FILTER. 

pressure  falls  too  rapidly  and  the  apparatus  requires 
continuous  attention  and  pumping.  The  pressure 
to  be  used  depends  on  the  denseness  of  the  mem- 
brane ;  two  or  three  atmospheres  (30  to  45  Ibs.  per 
square  inch)  will  generally  be  sufficient,  but  the 


FIG.  ii. 

apparatus  should  be  strong  enough  for  a  maximum 
working  pressure  of  five  atmospheres,  i.e.,  it  should 
be  tested  with  eight  atmospheres,  as  this  adds  very 
little  to  the  cost.  The  apparatus  is  supported  on  a 
tripod  stand,  and  the  beaker  or  flask  for  the  filtrate 
placed  below  it. 

The  membranes  are  made  from  circles  of  filter 
paper  impregnated  with  either  acetic  acid  collodion 


COLLODION  MEMBRANES.  71 

or  gelatin.  A  hard  filter  paper,  such  as  used  for 
vacuum  filtration  on  Buchner  funnels,  must  be 
employed ;  brands  equivalent  to  Schleicher  and 
Schuell's  Nos.  575  or  602  are  suitable. 

Collodion  membranes  are  more  convenient  than 
gelatin  ones ;  their  permeability,  i.e.,  the  average 
size  of  the  particles  which  are  just  retained,  varies 
with  the  concentration  of  the  collodion.  As  it  is 
difficult,  without  experience,  to  foretell  what  con- 
centration will  answer  in  any  given  case,  a  range  of 
filters  should  be  prepared  impregnated  with,  say, 
i>  2»  3>  5>  7  and  10  per  cent,  collodion,  i.e.,  sols  of 
collodion  cotton  containing  i,  2,  etc.,  gm.  of  cotton 
in  100  c.c.  of  sol,  the  solvent  being  glacial  acetic  acid. 
The  preparation  of  these  sols  has  been  fully  described 
under  "Dialysis/'  p.  24. 

If  a  number  of  filters  are  prepared  with  sols  of 
various  concentrations,  the  latter  should  be  marked 
in  pencil  on  the  disc  before  impregnation.  The  sol  is 
poured  into  a  small  dish — a  porcelain  developing 
dish  does  very  well, — a  filter  paper  seized  in  a  forceps 
near  the  edge,  within  the  width  which  will  eventually 
be  covered  by  the  rubber  joint  ring,  and  slowly 
immersed  in  the  sol  under  a  very  acute  angle  with 
its  surface.  Care  must  be  taken  not  to  trap  any  air 
bubbles  underneath  the  paper  and  to  have  it  uni- 
formly penetrated  by  the  sol ;  this  is  readily  seen  by 
the  paper  becoming  translucent,  like  oiled  paper, 
while  spots  not  penetrated  by  collodion  remain 
opaque  and  white.  The  thoroughly  impregnated 
disc  is  then  slowly  withdrawn  from  the  liquid  and 
held  vertically  above  it  to  allow  the  excess  to  drain 
off ;  it  should  be  turned  to  and  fro  in  its  own  plane, 
as  otherwise  a  thick  ridge  is  formed  at  the  bottom, 
which  may  prevent  a  good  airtight  joint  being  made 
when  the  disc  is  clamped  in  the  apparatus.  The 
discs  are  then  submerged  in  water,  which  is  con- 
stantly changed  until  afi  acetic  acid  has  been  washed 


72  WO.   OSTWALD'S 

out,  and  may  then  be  kept  indefinitely  in  water 
saturated  with  chloroform  or  camphor,  to  prevent 
the  formation  of  mould* 

Gelatin  sols  containing  from  2  to  10  per  cent,  may 
be  used  instead  of  collodion,  the  sols  being  prepared 
in  the  usual  way.  The  vessel  containing  the  sol 
during  impregnation  must  be  placed  in  a  water  bath, 
and  the  temperature  chosen  should  be  maintained 
constant  and  adhered  to  throughout,  as  otherwise 
filters  made  even  with  the  same  concentration  will 
vary  considerably.  The  discs  are  impregnated  and 
drained  as  described,  during  which  time  the  sol  sets 
to  gel ;  they  are  then  hardened  in  a  cold  solution  of 
formaldehyde,  2  to  4  per  cent.,  which  is  placed  in  a 
refrigerator  for  24  hours.  The  discs  are  then  rinsed 
in  water  and  can  be  kept  under  water  saturated  with 
chloroform.  //  either  collodion  or  gelatin  ultra-filters 
are  allowed  to  dry,  even  partially,  they  become  useless. 

Two  per  cent,  collodion  filters  should  give  a  colour- 
less filtrate  with  Prussian  blue  sol  (prepared  as 
described  on  p.  34),  while  3  to  4  per  cent,  filters 
should  retain  practically  the  whole  of  the  disperse 
phase  in  ordinary  gold  sols  (reduced  by  formaldehyde 
or  tannin). 

The  use  of  special  apparatus  and  the  consequent 
contact  with  metal  can  be  avoided  by  adopting 
either  of  the  following  methods  of  making  ultra- 
filters,  which  are  due  to  Wo.  Ostwald.  The  first  one 
furnishes  membranes  suitable  for  use  with  the  filter 
pump,  while  the  second  one  produces  "  spontaneous  " 
ultra-filters,  i.e.,  membranes  of  sufficient  permeability 
to  allow  liquid  to  pass  simply  by  hydrostatic  pressure. 

i.  Filters  for  Use  with  Vacuum. — The  collodion 
used  has  the  following  composition :  collodion 
cotton,  2  gm.  ;  alcohol,  14  c.c.  ;  ether,  84  c.c.  ;  or 
the  commercial  "  Collodion  P.B."  or  "  Collodion, 
methylated  "  may  be  used. 

For  preparing  conical  funnels,  a  circle  of  ordinary 


ULTRA-FILTERS.  73 

rough  filter  paper  is  folded  twice  in  the  usual  way 
and  placed  in  a  well-fitting,  smooth  glass  funnel. 
To  ensure  perfect  fit  it  may  sometimes  be  advisable 
to  make  one  fold  only  at  first,  to  place  the  paper  in 
the  funnel,  and  to  make  the  second  fold  when  good 
contact  all  round  has  been  secured  by  careful 
smoothing.  The  filter  thus  prepared  is  filled  with 
collodion  up  to  the  edge  ;  when  this  has  penetrated 
the  paper  over  the  entire  surface,  the  excess  is 
emptied,  the  filter  being  slowly  turned  while  this  is 
being  done.  Turning  is  continued  while  the  collo- 
dion dries  superficially  (the  time  required  depends  on 
the  room  temperature  and  the  desired  permeability, 
and  may  be  from  three  to  six  minutes)  ;  when  it  no 
longer  sticks  to  the  finger  on  being  touched  lightly, 
the  funnel  is  placed  in  distilled  water.  The  filter  is 
ready  for  use  after  about  15  minutes'  immersion,  but 
may  be  kept  under  water  indefinitely  provided 
chloroform  or  camphor  is  added. 

The  same  method  may  be  adopted  for  preparing 
Buchner  funnels,  but  in  this  case  a  joint  must  be 
made  between  the  edge  of  the  paper  and  the  per- 
forated plate  before  impregnation,  as  otherwise  the 
collodion  gets  under  the  paper.  A  solution  of  2  gm. 
of  white  crepe  rubber  in  100  c.c.  of  petroleum  ether 
is  used  for  the  purpose.  The  funnel  is  inclined  under 
an  angle  of  45°  and  a  few  cubic  centimetres  of  the 
rubber  sol  poured  down  the  side,  care  being  taken 
that  it  does  not  reach  the  perforations.  By  turning 
the  funnel  round  the  sol  is  distributed  to  form  a  band 
round  the  perforated  area,  and  while  this  is  still 
"  tacky  "  the  circle  of  filter  paper  is  placed  in  posi- 
tion and  squeezed  down  all  round.  After  a  few 
minutes'  drying  a  second  rubber  band  is  produced  in 
exactly  the  same  way,  and  when  this  has  dried  the 
paper  is  impregnated  with  collodion,  as  described 
above.  When  the  collodion  Tias  dried  to  nearly 
the  required  extent  a  third  rubber  joint  is  made, 

I 


74  WO.  OSTWALD'S 

and  the  funnel  placed  in  distilled  water,  as  before 
described. 

2.  Spontaneous  Filters. — The  collodion  for  these 
contains  4  gm.  of  collodion  cotton  to  12  c.c.  of 
alcohol  and  84  c.c.  of  ether.  A  circle  of  ordinary 
rough  filter  paper  is  folded  twice  in  the  usual  way, 
placed  in  a  well-fitting  smooth  funnel,  and  thoroughly 
wetted  with  distilled  water.  Any  excess  is  poured  off 
or  allowed  to  drain  through  ;  if  a  little  remains  in  the 
point  of  the  filter  it  must  be  removed  with  a  twisted 
spill  of  filter  paper.  The  funnel  is  then  partly  filled 
with  collodion,  which  is  spread  uniformly  by  inclining 
and  turning,  the  excess  poured  off,  and  the  collodion 
allowed  to  dry  for  four  to  five  minutes,  after  which 
a  second  layer  of  collodion  is  poured  in  the  same 
way.  When  this  has  dried  a  few  minutes  the  filter  is 
ready  for  use.  A  properly  made  filter  of  this  kind 
should  give  a  colourless  filtrate  with  a  gold  sol  made 
by  reduction  with  formaldehyde. 

Buchner  funnels  may  likewise  be  used,  no  rubber 
joint  being  necessary  in  this  case.  The  paper  is 
thoroughly  wetted  with  distilled  water  and  placed 
flat  on  the  perforated  plate.  Sufficient  collodion  is 
then  poured  on  to  cover  the  whole  of  the  paper,  and 
the  excess  poured  off,  leaving,  however,  a  remnant 
of  two  or  three  cubic  centimetres,  which  is  carefully 
distributed  round  the  edge  of  the  paper  by  inclining 
the  funnel  about  45°  and  turning  it  continuously 
until  the  collodion  no  longer  flows.  A  second  lot  of 
collodion  is  poured  in  exactly  the  same  way  after  the 
first  one  has  dried  to  the  desired  extent,  and  the  filter 
is  ready  for  use  when  this  is  sufficiently  dry.  Par- 
ticular attention  must  be  paid  to  getting  a  sufficient 
rim  of  collodion  round  the  edge  of  the  paper,  as  this 
makes  the  joint  and  prevents  the  liquid  from 
escaping  underneath  the  paper. 

An  extremely  convenient  method  of  making  small 
ultra-filters  consists  in  the  use  of  the  seamless 


ULTRA-FILTERS.  75 

extraction  thimbles,  which  can  be  obtained  in  a 
variety  of  sizes.  If  apparatus  for  using  them  with 
vacuum  is  available,  they  may  be  impregnated  dry, 
as  described  under  (i)  ;  otherwise  it  is  more  con- 
venient to  impregnate  them  wet  and  use  them  as 
spontaneous  filters  by  placing  them  in  a  loosely  - 
fitting  cylindrical  funnel. 

LITERATURE. 

For  sizes  of  particles  retained,  determination  of 
diameter  of  pores,  etc.,  see  the  original  papers  by  H. 
Bechhold,  Zeitschr.  f.  phys.  Chem.,  64,  328  (1908)  ;  Kott.- 
Zeitschr.,  II.,  3  and  33  (1907)  ;  III,  226  (1908). 


CHAPTER  X. 
OPTICAL  METHODS  OF  EXAMINATION. 

THE  simplest  and  most  sensitive  method  of  show- 
ing the  presence  of  disperse  matter  in  a  liquid  is 
examination  by  the  Tyndall  cone,  i.e.,  a  narrow 
beam  of  intense  light  projected  through  the  liquid 
and  viewed  at  right  angles  to  the  direction  of  the 
axis  of  the  beam.  The  liquid  to  be  examined  is 
placed  in  a  prismatic  cell ;  the  small  cemented 
specimen  cells  supplied  by  most  makers  of  apparatus 
are  suitable.  If  possible,  cells  cemented  with 
dichromate-gelatin  should  be  chosen,  as  they  can  be 
used  both  for  aqueous  solutions  and  organic  solvents. 
One  face  of  the  cell  is  covered  with  black  velvet,  or, 
better  still,  the  cell  is  placed  in  a  small  wooden  box, 
provided  with  two  circular  openings  (Fig.  12)  at 
right  angles  to  each  other  and  lined  with  black 
velvet  or  painted  a  dead  black  with  Indian  ink.  The 
light  from  a  small  hand-regulated  arc  lamp,  Nernst 
lamp  with  horseshoe  filament,  or  a  tungsten  arc 
("  point  o'  light  lamp  "),  is  projected  through  the 
cell  by  means  of  a  lens  so  placed  that  the  focus  falls 
about  the  middle  of  the  cell,  opposite  the  second 
opening,  through  which  the  path  of  the  beam  is 
viewed.  All  light  should  be  screened.  If  particles 
are  present  the  beam  is  visible  ;  when  the  disperse 
phase  is  colourless  (e.g.,  mastic  suspension)  the  cone 
shows  a  bluish  tinge  ;  when  it  is  coloured  the  beam 
may  show  a  colour  different  from  that  of  the  liquid 
viewed  in  transmitted  light  (e.g.,  red  gold  sols,  in 
which  the  cone  shows  green) .  It  must  be  remembered 


TYNDALL  CONE. 


77 


that  even  true  solutions  of  substances  of  high  mole- 
cular weight,  e.g.,  cane  sugar,  show  the  cone,  and  that 
it  is  not  entirely  absent  even  in  filtered  distilled 
water.  Water  freshly  ultra-filtered  and  collected 
with  due  precautions  against  contamination  by  dust 
is  nearly  "  optically  void,"  i.e.,  the  cone  is  invisible. 

By  placing  a  suitable  analyser,  say  a  Nicoll  prism 
mounted  in  a  collar  permitting  it  to  be  rotated,  in 
the  opening  A,  the  light  emitted  by  the  cone  can  be 
shown  to  be  polarized  ;  if  the  Nicoll  is  rotated  the 
intensity  of  the  cone  varies,  becoming  a  minimum  in 


FIG.  12. 


0  from  each  other, 
with  a  disperse 


a 

(mastic 


sus- 


two  positions  of  the  prism  at  180 
Complete  extinction  occurs  only 
phase  consisting  of  a  non-conductor 
pension) . 

Ultra-microscopic  and  Dark-ground  Examination. — 
The  "slit"  ultra -microscope  and  the  cardioid  con- 
denser require  large  arc  lamps  and  special  wiring,  and 
are  beyond  the  scope  of  this  book  ;  those  in  a  position 
to  use  them  will  obtain  all  necessary  instructions 
from  the  makers.  Real  ultra-microscopic  illumina- 
tion is  provided  by  the  Jentzsch  ultra- condenser,  a 
section  through  which,  showing  the  path  of  the  rays, 
is  shown  in  Fig.  13.  It  has  the  advantage  of  requiring 


78 


ULTRA-CONDENSER. 


only  a  4  to  5  ampere  hand-regulated  arc  lamp,  of 
holding  a  comparatively  large  volume  of  liquid,  so 
that  adsorption  effects  are  minimized,  and  of  being 
very  easily  centred.  The  optical  part  is  cemented 
into  a  cylindrical  metal  casing,  which  is  closed  by  a 
metal  cover  provided  with  bayonet  joint  and  a 
central  quartz  window  for  observation.  The  ultra- 
condenser  is  placed  on  the  stage  of  the  microscope  so 
that  the  spigot  on  its  lower  side  fits  the  opening  in 
the  former — the  ordinary  condenser  being,  of  course, 
removed  and  the  plane  mirror  used  for  illumination. 

The  dimensions  of  the 
ultra  -  condenser  do  not 
permit  the  use  of  objec- 
tives of  shorter  focal 
length  than  6  mm.  (or 
J"),  but,  as  the  images  are 
not  geometrical,  there  is 
no  limit  to  the  eyepiece 
magnification  permissible, 
and  the  highest  power 
eyepiece  available  may 
be  used. 

The  condenser  is  filled 
with  the  liquid  to  be  examined  by  means  of  the  inlet 
and  outlet  branches  provided  on  the  cover,  care  being 
taken  not  to  leave  an  air  bubble  at  the  top  of  the  liquid 
under  the  quartz  cover.  The  light  from  the  arc  lamp, 
which  should  be  provided  with  a  lens  giving  a  nearly 
parallel  beam,  is  then  directed  on  the  plane  mirror, 
which  it  should  fill  completely  and  uniformly,  and 
the  condenser  placed  in  position.  The  light  is  now 
centred,  using  the  ¥  objective  and  a  low  power  eye- 
piece, by  adjusting  the  mirror  until  the  brightly 
illuminated  spot  is  exactly  in  the  centre  of  the  field. 
The  low-power  eyepiece  is  then  replaced  by  the 
highest  power  available ;  Zeiss's  No.  18  compen- 
sating eyepiece,  or  an  equivalent,  is  suitable.  The 


FIG.  13. 


DARK-GROUND  CONDENSERS.    79 

most  highly  illuminated  layer  will  be  easily  found  by 
focussing  up  and  down,  and,  as  this  layer  is  at  some 
distance  from  any  boundary  surface,  the  Brownian 
movement  will  be  seen  in  great  perfection.  The 
large  diffraction  rings  which  appear  and  disappear 
round  many  particles  indicate,  of  course,  vertical 
movement  out  of  the  focal  plane. 

Coagulation  of  sols  can  be  very  conveniently 
studied  with  this  condenser  by  running  in  an  elec- 
trolyte solution  through  one  of  the  branches  on  the 
cover,  or  by  adding  a  small  amount  of  coagulant  to 
the  sol  before  it  is  filled  into  the  apparatus.  After  use 
the  condenser  must  be  carefully  washed  with  dis- 
tilled water  and  thoroughly  dried  with  linen  free 
from  grease.  This  applies  equally  to  the  metal 
parts  in  contact  with  the  liquid. 

If  the  ultra-condenser  is  not  available,  the  pre- 
sence of  at  least  coarser  ultra-microscopic  particles 
can  be  detected  by  means  of  one  of  the  numerous 
dark-ground  condensers.  Typical  forms  of  this 
apparatus  are  the  Zeiss  "  Paraboloid  "  condenser, 
the  Reichert  "Table"  condenser,  and  the  Jentzsch 
"  Concentric "  condenser,  which  latter,  like  the 
Jentzsch  ultra-condenser,  is  now  made  in  this 
country.  The  methods  of  using  and  centring  are 
slightly  different  with  the  different  types,  and  are 
generally  adequately  described  in  the  makers' 
pamphlets.  Since  they  all  depend  on  total  reflection 
at  the  cover  glass,  slides  of  the  thickness  prescribed 
by  the  makers  must  be  used  in  all  cases.  Both 
slides  and  cover  glasses  must  be  carefully  cleaned  in 
the  following  manner.  They  are  washed  in  hot 
dichromate-sulphuric  acid  mixture  for  five  to  ten 
minutes  and  then  rinsed  thoroughly  with  distilled 
water.  The  slides  or  cover  glasses  are  then  seized, 
one  by  one,  with  a  spring  forceps  and,  after  draining 
off  the  bulk  of  the  water,  placed  in  strong  alcohol, 
in  which  they  are  kept  until  required.  Immediately 


8o       METHOD   OF  CLEANING  SLIDES. 

before  use  the  slide  is  withdrawn  from  the  alcohol 
by  seizing  one  corner  with  a  spring  forceps,  and  the 
adhering  alcohol  burnt  off  over  a  spirit  lamp  or 
Bunsen  burner.  As  soon  as  the  slide  has  cooled  it 
is  placed  on  the  condenser,  ample  cedar  oil  being 
used,  as  explained  in  the  descriptive  pamphlets. 
The  cover  glasses  may  be  treated  in  the  same  way, 
provided  they  do  not  crack  too  frequently  ;  if  this 
should  be  the  case,  the  alcohol  may  simply  be 
evaporated  at  a  sufficient  distance  from  the  flame  to 
prevent  ignition.  Before  the  cover  glass  is  made 
ready  a  large  drop  of  sol,  free  from  any  air  bubble, 
should  be  placed  on  the  centre  of  the  slide,  and  the 
cover  glass  dropped  gently  on  it  immediately  it  has 
cooled.  The  essential  point  of  the  method  described  is 
that  slides  and  cover  glasses  are  not  touched  with  the 
fingers  or  with  any  textile  material,  as  this  renders  them 
entirely  useless  for  ultra-microscopic  work. 

The  layer  of  liquid  between  cover  glass  and  slide 
is,  of  course,  of  very  slight  depth,  and  careful  focus- 
sing on  its  central  portion  is  necessary  to  observe 
particles  moving  freely.  Many  particles  will  always 
be  found,  by  suitable  focussing,  to  have  adhered  to 
the  two  glass  surfaces.  Electrolyte  coagulation  can 
be  observed,  though  in  a  somewhat  rough  fashion, 
by  placing  a  drop  of  solution  on  the  edge  of  the  cover 
glass,  so  that  it  can  diffuse  into  the  bulk  of  liquid. 
The  most  convenient  objects  for  becoming  familiar 
with  the  use  of  the  apparatus  are  comparatively 
coarse  systems,  especially  mastic  or  gamboge  sus- 
pensions. 

LITERATURE. 

Full  information  on  the  various  types  of  condensers  is 
to  be  found  in  the  pamphlets  issued  by  the  makers 
(Zeiss,  Leitz  and  Chas.  Baker).  Photometric  investiga- 
tion on  Tyndall  cone,  connection  between  size  of  particles 
and  luminosity,  etc.,  by  W.  Mecklenburg,  Koll.-Zeitschr., 

xiv.,  172  (1914) ;  xv.,  149  (i9M) ;  XVL,  97 


CHAPTER  XI. 
CATAPHORESIS. 

A  SIMPLE  apparatus,  suitable  for  practice  and  pre- 
liminary work,  may  De  made  up  from  glass  parts 
available  in  every  laboratory  in  the  manner  illus- 
trated in  Fig.  14.  A  U-tube,  about  250  mm.  long, 
provided  with  an  inlet  tube  at  the  lowest  point  of 
the  bend,  is  supported  in  a  suitable  stand.  The 
inlet  tube  is  connected  by  a  rubber  tube,  about 
350  mm.  long,  to  a  funnel  capable  of  holding  about 
75  c.c.  The  tube  is  fitted  with  a  screw  clip  or  with 
one  of  the  patent  clips  provided  with  a  catch,  which 
allows  it  to  be  left  fully  opened,  near  the  end  of  the 
inlet  tube. 

Two  electrodes  are  inserted  in  the  tops  of  the  limbs, 
consisting  of  foil  rolled  into  a  cylinder,  the  diameter 
of  which  should  be  about  2  mm.  less  than  that  of 
the  tube.  Platinum  is,  of  course,  the  best  material, 
failing  which,  silver  ;  even  cylindrical  solid  carbon 
electrodes  may  be  employed,  but  small  particles  are 
liable  to  become  detached  from  the  latter  during 
use.  The  electrodes  are  fixed  to  stout  wires,  which 
are  best  mounted  in  a  strip  of  ebonite,  acting  also  as 
distance  piece,  and  provided  with  terminals. 

The  apparatus  is  charged  in  the  following  manner. 
The  clip  is  opened  and  the  sol  to  be  examined  poured 
into  the  funnel,  the  latter  being  held  so  that  its  edge 
is  about  10  mm.  above  the  bottom  of  the  bend.  The 
liquid  should  just  reach  the  latter  ;  the  funnel  is 
then  lowered  and  again  raised  to  the  original  level, 
to  drive  out  any  air  which  may  have  been  trapped 

L.M.  6 


82 


U-TUBE  METHOD. 


in  the  rubber  tube,  and  the  cock  closed,  with  the  sol 
standing  just  at  the  bottom  of  the  U-tube.  Distilled 
water  is  now  poured  into  the  latter  so  as  to  fill  the 
limbs  to  about  half  their  height.  The  funnel  is  then 

raised  until  the  level  oj 
the  liquid  in  it  is  about 
i  mm.  below  that  of  the 
water  in  the  limbs,  and 


1L  A  the  cock  opened  full  bore. 

J  J     The  funnel  is  now  very 

slowly  raised,  the  sol 
flows  into  the  U-tube, 
and  the  level  of  the 
water  in  the  limbs  rises 
correspondingly.  The 
funnel  must  be  raised 
at  the  same  rate,  i.e., 
the  level  of  the  sol  in 
the  funnel  should  never 
be  more  than  i  or 
/  /  2  mm.  above  the  water 

I         J    I       I    I  level  in  the  limbs.     If 

^ — S  /       /    /  this  is  done  properly, 

the  sol  rises  in  the  U 
without  mixing  with 
the  water,  and  even- 
tually shows  a  sharp 
boundary  in  both  limbs. 
If  the  funnel  is  raised 
too  rapidly,  or  if  the 
cock  is  not  fully  opened, 
the  sol  issues  in  a  jet, 
impinges  on  the  upper 
wall  of  the  U-bend,  and  rises  unequally  in  the  limbs, 
without  filling  them  completely  and  without  forming 
the  necessary  sharp  boundary  surface  against  the 
water.  While  it  is  quite  easy  to  fill  the  tube  properly 
oftfir  a.  littlfi  nract.ice.  the  bednner  will  find  the  pro- 


FIG.  14. 


a  little  practice,  the  beginner 


U-TUBE  METHOD.  83 

cedure  much  facilitated  by  a  loose  plug  of  carefully 
washed  cotton  or  glass  wool,  placed  in  the  inlet  tube 
at  its  junction  with  the  bend.  This  checks  and  dis- 
tributes the  admission  of  the  sol  and  prevents  its 
issuing  in  a  jet. 

Sufficient  sol  must,  of  course,  be  admitted  to  raise 
the  water  level  in  the  limbs  so  far  that  the  electrodes 
are  covered  completely.  When  this  is  the  case  they 
may  be  connected  to  the  electric  supply  ;  the  light- 
ing supply  may  be  used,  of  course  with  sufficiently 
insulated  wires  or  flexible  leads.  With  a  voltage  of 
200  the  gradient  in  a  tube  of  the  dimensions  described 
above  is  about  5V/cm.,  so  that  a  very  distinct  shifting 
of  the  boundary  is  noticeable  after  10  minutes.  The 
polarity  of  the  electrodes  must,  of  course,  be  deter- 
mined, e.g.,  with  the  "  pole  finding  paper  "  obtain- 
able for  this  purpose.  Rough  measurements  of  the 
rate  of  travel  may  be  made  by  marking  the  original 
boundary  and  measuring  the  displacement  (choosing 
whichever  boundary  is  the  sharper)  after  a  definite 
time.  The  field  strength  is  the  voltage  divided  by 
the  distance  of  the  electrodes  ;  the  latter  is  somewhat 
uncertain,  but  the  total  distance  in  the  axis  of  the 
tube,  i.e.,  twice  the  length  of  the  straight  limb,  from 
the  lower  edge  of  the.  electrode,  plus  half  the  arith- 
metical mean  of  the  internal  and  external  circum- 
ference of  the  bend,  may  be  taken  as  approximately 
correct. 

For  exact  work  a  specially  made  apparatus  is 
preferable,  which  incidentally  avoids  the  use  of 
rubber  connections.  A  convenient  form  (after 
W.  Nernst  and  A.  Coehn)  is  illustrated  in  Fig.  15. 
It  consists  of  a  U-tube  provided  with  two  large 
cocks  at  the  junctions  of  the  straight  limbs  with  the 
bend.  An  inlet  tube,  about  3  or  4  mm.  diameter, 
leads  into  the  lowest  point  of  the  latter,  and  is  bent 
at  right  angles  to  the  plane  of  the  drawing,  ter- 
minating at  the  top  in  a  charging  funnel  of  suitable 

a— 2 


84 


U-TUBE  METHOD. 


capacity.  A  scale  of  millimetres  (not  cubic  centi- 
metres /)  may  be  etched  on  the  limbs.  The  limbs  are 
provided  with  electrodes  such  as  described  above. 

The  apparatus  is 
charged  as  follows. 
The  funnel  is  rilled 
with  the  sol  to  be 
examined,  the  cocks 
A  and  A'  opened,  and 
then  the  cock  B,  until 
the  sol  just  rises 
above  the  large  cocks. 
B,  and  subsequently 
A  and  A'  also,  are 
closed,  and  the  small 
amount  of  sol  in  the 
limbs  removed  with 
spills  of  filter  paper. 
The  limbs  are  then 
filled  to  the  same 
height  with  distilled 
water  and  the  elec- 
trodes placed  in  posi- 
tion. The  cocks  A 
and  A'  are  now 
opened,  and  then  B, 
which  must  be  done 
very  slowly  and  uni- 
formly. The  sol  and 
the  supernatant 
water  in  the  limbs 
now  rise ;  sol  is 
admitted  until  the 
electrodes  are  sub- 
merged, when  they 
may  be  connected  to 
the  supply.  What 


R 

Joj 

i 

B 

Q 

o 

V 

^ 

o 

i 

J 

A 


B 


FIG.  15. 


has  previously  been 


MICROSCOPIC  METHOD.  85 

said  regarding  the  electric  gradient,  of  course,  applies 
equally  to  this  form  of  apparatus. 

If  measurements  of  the  velocity  of  cataphoresis 
are  made,  the  results  are  usually  reduced  to  unit 
gradient,  as  stated.  To  give  an  example,  the 
boundary  travels  to  the  anode,  the  displacement 
amounting  to  22  mm.  in  10  minutes.  The  velocity 
per  second  is  accordingly  2*2/600  cm.  =  0-00366 
=  366  x  io~5.  With  a  voltage  of  240  V  and  a 
distance  of  24  cm.  between  the  electrodes  the 
gradient  is  240/24  =  10  V/cm.  The  velocity  reduced 
to  unit  gradient  is,  therefore,  366/10  X  io~5  = 
36  X  io~5,  which  is  a  normal  value  for  the  more 
highly  dispersed  gold  sols. 

Microscopic  Observation  and  Measurement  of  Cata- 
phoresis.— Any  one  of  the  "  dark-ground  "  con- 
densers described  in  the  chapter  on  optical  methods 
of  examination  may  be  employed.  A  slide  must  be 
provided  with  electrodes  having  parallel  edges, 
between  which  the  liquid  is  contained.  The  elec- 
trodes are  two  strips  of  metal  foil ;  platinum  is,  of 
course,  preferable,  failing  which  silver  may  be  used. 
The  strips  should  be  3  mm.  wide  and  about  35  to 
40  mm.  long  for  the  standard  microscope  slide, 
i"  X  3".  They  are  fastened  parallel  to  each  other 
and  at  right  angles  to  the  length  of  the  slide,  so  that 
the  distance  between  the  outside  edges  is  equal  to 
the  width  of  the  cover  glass  to  be  used.  A  f "  cover 
will  be  found  convenient,  this  making  the  distance 
between  the  outside  edges  of  the  strips  about  22  mm. , 
and  the  distance  between  the  inside  edges,  i.e.,  the 
distance  between  electrodes,  about  16  mm. 

The  strips  are  fastened  to  the  slide,  which  has 
previously  been  thoroughly  cleaned  in  the  manner 
already  described,  with  Chatterton's  compound  (a 
preparation  made  for  insulating  purposes).  A  small 
piece  of  the  preparation  is  warmed  sufficiently  and 
drawn  out  into  a  thin  filament  about  i  mm.  diameter. 


86  MICROSCOPIC  METHOD. 

The  slide  and  the  two  strips  of  foil  are  then  placed  on 
a  warm  metal  surface  and  a  piece  of  the  filament 
about  25  mm.  long  laid  on  each  strip,  centrally  as 
regards  width,  and  at  one  end  of  the  strip,  so  as  to 
leave  a  length  of  10  to  15  mm.  clear.  The  strips, 
as  soon  as  the  filaments  of  compound  have  softened, 
are  picked  up  with  a  forceps,  inverted  and  placed 
parallel  to  each  other  on  the  slide  the  requisite  dis- 
tance apart  (the  position  of  the  outside  edges  can 
be  previously  marked  with  a  diamond  or  a  drawing- 
pen).  They  should  be  dropped  down  into  the  correct 
position  without  subsequent  shifting,  to  avoid 
smearing  the  slide  with  the  compound.  As  soon  as 
the  strips  are  in  position  the  slide  is  removed  from 
the  warm  surface  and  placed  on  a  clean  piece  of 
filter  paper  resting  on  a  wood  or  glass  table  ;  the 
strips  are  then  weighted  by  placing  them  on  a  second 
microscope  slide  and  on  this  a  50-gm.  weight. 
Sufficient  time — from  10  to  20  minutes  according  to 
the  temperature  of  the  room — must  be  allowed  for 
the  complete  hardening  of  the  cement. 

Electric  connection  to  the  electrodes  may  be  made 
simply  by  pressing  the  wires  down  on  the  projecting 
ends  of  the  electrodes,  of  course  taking  care  to 
insulate  the  latter  from  the  stage  of  the  microscope 
by  a  piece  of  thin  sheet  rubber  or  the  like.  For 
repeated  use  it  is,  however,  more  convenient  to  solder 
leads  of  thin  flexible  cord  to  the  ends  of  electrodes, 
the  opposite  ends  being  provided  with  terminals  kept 
apart  by  a  distance  piece  of  ebonite.  The  flexible 
cords  should  be  of  sufficient  length  to  allow  the 
terminals  to  rest  on  the  table,  out  of  the  way  of  the 
mirror,  when  the  slide  is  in  position  on  the  stage  of 
the  microscope. 

The  slide  is  placed  on  the  stage  and  optical  con- 
nection made  with  the  particular  condenser  used  in 
the  manner  prescribed  for  it.  The  slide  should,  if 
possible,  be  clamped  down  to  avoid  accidental 


MICROSCOPIC  METHOD.  87 

shifting.  A  large  drop  of  the  liquid  is  then  placed 
in  the  centre  of  the  space  between  the  electrodes ; 
this  should  be  of  sufficient  size  to  cover  the  field 
between  the  edges  of  the  two  strips,  and  make  good 
contact  with  both,  when  the  cover  glass  is  put  on. 
The  microscope  is  then  focussed  on  the  central  layer 
of  liquid,  so  as  to  observe  particles  which  are  moving 
freely,  and  the  current  turned  on.  In  view  of  the 
short  distance  between  the  electrodes  a  supply  at 
4  to  5  V  is  sufficient  (say,  from  accumulators  or  dry 
cells).  The  velocity  may  be  measured  by  means  of 
an  eyepiece  micrometer  and  stop-watch.  Release 
the  latter  when  a  particle  under  observation  passes 
through  one  of  the  numbered  divisions,  and  arrest 
the  watch  when  it  has  travelled,  say,  through  five 
divisions.  The  actual  value  of  the  micrometer 
reading  must,  of  course,  be  known,  or  determined 
in  the  usual  way  with  a  stage  micrometer.  The 
potential  gradient  is  :  voltage  at  terminals/distance 
of  electrodes  in  centimetres.  To  obtain  the  velocity 
of  the  particles  in  unit  gradient,  i.e.,  one  volt  per 
centimetre,  divide  the  velocity  actually  found  by 
the  gradient.  For  instance,  a  particle  is  found  to 
travel  five  divisions  of  the  scale  in  32  seconds.  Five 
divisions,  with  the  particular  objective  and  eyepiece 
used,  correspond  to,  say,  0-24  mm.  =  0*024  cm.  = 
24  X  io~8  cm.  The  velocity  per  second  is,  there- 
fore, 24/32  x  io~8  =  75  x  io~6  cm.  With  a  vol- 
tage of  four  and  a  distance  between  the  electrodes  of 
1-6  cm.,  the  gradient  is  4/1*6  =  2*5  V/cm.  To 
obtain  the  velocity  in  unit  gradient,  the  figure  found 
above  must,  therefore,  be  divided  by  2*5,  so  that  we 
finally  obtain  the  velocity  per  second  in  a  field  of 
one  volt  per  centimetre  =  75/2*5  X  10  - 5  =  30  X 
io~5  cm.  This  is  a  normal  value  for  a  metal  sol. 

The  microscopic  method  is  particularlyf convenient 
and  rapid  for  determining  the  sign  of  the-f  electric 
charge  when  only  small  quantities  of  the  liquid 


88  MICROSCOPIC  METHOD. 

under  examination  are  available.  When  using  it 
for  this  purpose,  remember  that  the  image  is  reversed, 
so  that,  if  the  anode  is  on  the  right,  negatively 
charged  particles  will  travel  to  the  left  in  the  field 
of  vision.  Measurements,  however,  may  be  subject 
to  fairly  considerable  errors  unless  a  layer  of  par- 
ticles is  observed  which  moves  freely  and  beyond 
the  influence  of  the  two  glass  surfaces.  A  cell 
designed  to  minimize  the  sources  of  error  has  recently 
been  described  by  Th.  Svedberg,  a  reference  to  which 
will  be  found  in  the  literature  given  below. 

LITERATURE. 

For  the  microscopic  method  generally,  see  A.  Cotton, 
and  H.  Mouton,  "  Les  ultramicroscopes,  etc.,"  Paris 
(1906).  For  elimination  of  errors,  Th.  Svedberg  and 
H.  Andersen,  KolL-Zeitschr.,  XXIV.,  155  (1919). 


CHAPTER  XII. 

ELECTROLYTE  PRECIPITATION  OF 
SUSPENSOID  SOLS. 

BEFORE  starting  experimental  work  the  student 
should  commit  to  memory  a  few  typical  figures  for 
the  concentrations  of  uni-,  bi-  and  tri-valent  ions 
which  produce  precipitation  in  suspensoid  sols. 
The  following  are  representative  : — 


Sol. 

Sign  of 
charge. 

Precipitation  concentration  in  millimoles/litre. 

AS2S3    . 
Pt 
Mastic 
Fe(OH)8 

Negative 
Positive 

NaCl    51-0 
NaCl      2-5 
NaCl    1,000 
NaCl       9-25 

CaCl2     0-65 
BaCl2     0-06 
CaCla    25 
K2SO4    0-20 

A1C13   0-09 
A1C13   o-o  i 
AlCla   0-2 

These  figures  give  the  concentration  existing  in 
the  mixture  of  sol  and  electrolyte  ;  if,  e.g.,  18  c.c.  of 
the  As2S3  sol  is  to  be  coagulated  by  2  c.c.  of  NaCl 
solution,  the  latter  will  have  to  contain  510  millimoles 
of  NaCl,  since  by  the  addition  of  the  sol  it  is  diluted 
to  one-tenth  of  its  original  concentration. 

A  second  point  to  be  noted  is  the  difference  in  the 
corresponding  values,  say,  for  NaCl,  between  the 
three  negative  sols.  This  may  be  specific,  i.e.,  a 
given  sol  may  be  much  more  stable  to  electrolytes 
generally  than  others  ;  such  is  the  case  with  the 
mastic  suspension  according  to  different  observers. 
The  precipitation  concentration,  however,  also 
depends  on  the  concentration  of  the  sol,  and  this  partly 


go        ELECTROLYTE  PRECIPITATION. 

explains  the  difference  between  the  As2S3  and  the 
Pt  sol,  the  former  being  much  more  concentrated. 

The  figures  for  the  As2S3  sol  may  be  taken  as 
typical  for  this  and  for  the  (dilute)  Prussian  blue 
sol,  as  described,  while  the  figures  for  the  Pt  sol  will 
be  found  approximately  correct  for  the  gold  sol 
made  by  the  formaldehyde  method.  The  Prussian 
blue  sol  will  be  found  the  most  convenient  for  the 
first  experiments. 

The  next  point  to  be  considered  is  the  method  of 
adding  the  electrolyte  solution  to  the  sol.  A  number 
of  the  classical  investigations  were  carried  out  by 
titration,  i.e.,  by  adding  electrolyte  solution  to  the 
sol  until  perceptible  coagulation  took  place.  This 
method  has  the  drawbacks  that,  unless  there  is  a 
very  marked  colour  change,  as  with  red  gold  sols,  it 
is  by  no  means  easy  to  notice  an  exact  end-point, 
that  it  excludes  the  time  factor,  and  that  the  con- 
centration of  sol  varies  with  different  amounts  of 
coagulating  solution.  Nevertheless  the  method 
gives  a  rough  idea,  and  may  be  used  for  preliminary 
trials,  care  being  taken  to  use  fairly  large  volumes 
of  sol  and  to  place  the  beaker  containing  it  during 
titration  so  that  small  changes  in  colour  or  turbidity 
can  be  readily  noticed. 

The  procedure  to  be  adopted  for  exact  deter- 
minations is  as  follows  :  a  uniform  volume  of  sol  is 
fixed  upon,  to  which  is  added  a  definite  fraction  of 
coagulating  solution,  the  concentration  of  which  is 
varied.  In  this  way  the  sol  concentration  is  kept 
uniform.  The  sol  and  solution  are  mixed  by  a 
uniform  procedure,  say  closing  the  test  tube  con- 
taining it  and  reversing  it  twice  or  four  times  ;  the 
mixture  is  allowed  to  stand  for  a  definite  time,  say 
two  or  three  hours,  and  is  then  examined. 

Eighteen  c.c  of  sol  and  2  c.c.  of  solution  will  be 
found  convenient,  the  latter  being  diluted  by  the  sol 
to  one-tenth  of  its  original  concentration.  As^about 


ELECTROLYTE  PRECIPITATION.        9I 

51  millimoles  of  NaCl,  0-65  of  CaCl2,  and  0-09  of 
A1C13  respectively  are  the  concentrations  required 
in  the  mixture  to  produce  precipitation,  the  concen- 
trations of  the  solutions  used  will  have  to  be  ten  times 
greater,  viz.,  510  millimoles  of  NaCl,  6-5  of  CaCla, 
and  0-9  of  A1C13. 

Since  these  concentrations  just  produce  precipita- 
tion in  a  certain  sol,  it  will  be  desirable  to  have  a 
considerable  margin,  and  the  following  three  stan- 
dards for  negative  sols  should  be  prepared  : — 

NaCl      .  1,000  millimoles  =  58*5     gm.  in  a  litre. 
CaCl2     .       15         „         =    1-665  gm. 
A1C13      .         2         „         =    0-267  gm. 

For  very  accurate  work,  or  if  the  materials  are  of 
doubtful  purity,  these  solutions  should  be  stan- 
dardized against  suitable  standard  solutions. 

Since  one  part  of  these  solutions  added  to  nine 
parts  of  sol  will  certainly  produce  coagulation,  more 
dilute  solutions  of  known  strength  must  be  prepared. 
Do  this  by  mixing  in  test  tubes,  say,  8  c.c.  of  solution 
with  2  c.c.  of  water,  6  c.c.  of  solution  with  4  c.c. 
of  water,  4  c.c.  of  solution  with  6  c.c.  of  water, 
and  2  c.c.  of  solution  with  8  c.c.  of  water ;  label 
the  tubes  0-8,  0-6,  0-4  and  0-2,  these  being  their 
respective  concentrations  referred  to  the  stock 
solutions. 

Now  place  18  c.c.  of  the  sol  to  be  examined  in  each 
of  five  test  tubes,  label  them  i,  0-8,  0-6,  0-4  and  o»2, 
add  to  each  2  c.c.  of  the  corresponding  solution,  mix 
by  the  standard  method  decided  upon,  and  allow 
the  tubes  to  stand  for  a  definite  time,  say  two  or 
three  hours.  It  will  then  be  found,  e.g.,  that  the 
contents  of  i,  0-8  and  o«6  have  been  precipitated, 
but  that  those  of  0*4  and  0-2  have  not  changed.  The 
limit  concentration  accordingly  lies  between  the 
electrolyte  concentrations  prevailing  in  0-6  and  0*4. 
The  solutions  added,  if  NaCl  was  used,  contained 


92        ELECTROLYTE  PRECIPITATION. 

1,000  x  0-6  and  1,000  x  0-4,  i.e.,  600  and  400  milli- 
moles  respectively  ;  since  they  were  diluted  to  one- 
tenth  by  the  sol,  the  actual  concentrations  in  the 
mixture  are  60  and  40  millimoles.  The  minimum 
concentration  necessary  for  precipitation  lies  between 
these  two  ;  to  determine  it  more  accurately,  an 
intermediate  concentration  of  the  added  solution 
may  now  be  tried,  say  0-5  of  the  original.  For  this 
purpose  mix,  say,  2  c.c.  of  stock  solution  with  2  c.c. 
of  water,  and  add  2  c.c.  of  the  mixture  to  18  c.c.  of 
sol  as  before.  If  precipitation  just  occurs  within  the 
standard  time,  the  limit  concentration  is  obviously 
1,000  X  0-5/10  =  50  millimoles  per  litre. 

The  procedure  just  described  should  be  carried  out 
with  several  sols,  e.g.,  Prussian  blue,  arsenic  tri- 
sulphide  and  gold  reduced  by  formaldehyde,  with  all 
three  electrolytes,  and  the  results  tabulated.  The 
results  should  be  compared  with  the  numerous  data 
given  in  the  literature  and  carefully  checked  if  they 
show  very  marked  deviations  from  the  average. 
The  contents  of  the  sols  in  disperse  phase  should  also, 
for  comparison,  be  calculated  from  the  data  given 
for  their  preparation. 

A  similar  procedure  should  be  adopted  with  the 
ferric  hydroxide  sol,  the  only  representative  of  the 
positively  charged  sols.  By  reference  to  the  table  at 
the  beginning  of  the  chapter  we  find  that  the  limit 
concentrations  are  9-25  millimoles  of  NaCl  and 
0-20  of  K2S04  (Na2S04  may  be  used  instead). 
Using  the  same  ratio  as  before,  2  c.c.  of  solution  to 
18  c.c.  of  sol,  the  concentrations  of  the  former  will  be 
92*5  and  2-0  millimoles,  and  to  have  the  same  margin 
as  before  we  shall  require  stock  solutions  of  the 
following  concentrations  : — 

NaCl      .     200  millimoles  =  11-7  gm.  in  a  litre. 
Na2S04  4         „         =    0-568  gm. 

The  NaCl  solution  may,  of  course,  be  made  up  from 


ELECTROLYTE  PRECIPITATION.        93 

that  previously  used  for  negative  sols  by  diluting 
with  four  volumes  of  water. 

LITERATURE. 

For  electrolyte  precipitation  generally,  W.  D.  Ban- 
croft, Second  Report  of  British  Association  Committee 
on  Colloid  Chemistry,  1918,  p.  2.  A  very  complete  study 
of  fractional  precipitation  in  Sven  Oden,  "  Der  kolloide 
Schwefel,"  N.  A.  Reg.  Soc.  Scient.  Upsal.,  Sen  IV., 
Vol.  3,  No.  4  (1913).  Coagulation  velocity :  H.  H. 
Paine,  Koll.-Zeitschr.,  XL,  115  (1912)  ;  H.  Freundlich, 
and  C.  Ishizake,  Faraday  Soc.  Gen.  Discussion  on 
Colloids  and  their  Viscosity,  1913 ;  H.  R.  Kruyt  and  J. 
van  der  Speck,  KolL-Zeitschr.,  XXV.,  I  (1919),  a  very 
careful  study  of  electrolyte  coagulation. 


CHAPTER  XIII. 

MUTUAL  PRECIPITATION   OF   SUSPENSOID 
SOLS. 

SOLS  in  which  the  disperse  phases  carry  opposite 
charges  precipitate  each  other  when  mixed  in 
definite  ratios,  while  no  precipitation  occurs  if  an 
excess  of  either  sol  is  present. 

The  ferric  hydroxide  sol  described  above  will, 
generally  speaking,  precipitate  an  equal  volume  of 
the  Prussian  blue,  the  gold  sol  reduced  by  formalde- 
hyde, or  the  mastic  suspension.  Place  5  c.c.  of  each 
of  the  negative  sols  into  test  tubes,  add  to  each  5  c.c. 
of  the  (dialysed  !)  ferric  hydroxide  sol,  mix  by  a 
uniform  procedure,  and  allow  the  tubes  to  stand. 
The  Prussian  blue  and  the  gold  sol  will  generally 
show  coagulation  within  a  few  minutes,  while  the 
mastic  suspension  may  take  15  to  25  minutes.  The 
coagulum  contains  both  disperse  phases,  so  that  the 
liquid  in  the  test  tubes  is  colourless  after  the  former 
has  settled. 

If  precipitation  fails  to  occur  with  any  of  the  sols 
mentioned,  or  with  any  other  negative  sol  mixed 
with  an  equal  volume  of  ferric  hydroxide  sol,  the 
correct  ratio  must  be  ascertained  by  experiment. 
For  this  purpose  place  in  test  tubes  i,  2,  3,  etc.,  up 
to  9  c.c.  of  ferric  hydroxide  sol  and  add  (in  the  same 
order)  9,  8,  7,  etc.,  down  to  i  c.c.  of  the  negative  sol. 
The  contents  of  each  tube  must  be  mixed,  by  a 
uniform  procedure,  immediately  after  the  second  sol 
has  been  added.  After,  say,  one  hour  note  the  ratio 
in  the  tube  or  tubes  in  which  coagulation  has  occurred. 


MUTUAL  PRECIPITATION  OF  SOLS.     95 

Examine  the  electrical  condition  of  two  mixtures 
in  which  no  coagulation  has  occurred,  one  having 
ferric  hydroxide,  and  the  other  negative  sol  in  excess. 
For  this  purpose  note  the  ratios  and  then  make  up 
a  sufficient  quantity  of  the  mixtures  for  cataphoresis 
in  the  U-tube.  The  sign  of  the  charge  in  the  mixture 
will  be  found  to  be  that  of  the  sol  present  in  excess, 
the  charge  on  the  other  sol  having  been  reversed. 


CHAPTER  XIV. 

PROTECTION. 

THE  protective  effect  of  emulsoids  may  be  demon- 
strated in  two  ways.  The  emulsoid  may  be  added 
to  one  component  of  a  reaction  which  produces  a 
precipitate  and  may  cause  the  latter  to  become  much 
more  highly  disperse  than  it  would  be  in  a  pure 
aqueous  medium.  Or  the  emulsoid  may  be  added  to 
an  existing  sol,  in  which  case  it  protects  it  from  electro- 
lytes, i.e.,  the  concentration  of  the  latter  necessary 
to  produce  coagulation  is  considerably  increased. 

To  demonstrate  the  formation  of  a  highly  disperse 
precipitate  in  the  presence  of  a  protective  colloid, 
dissolve  0-5  gm.  of  crystallized  barium  chloride  in 
50  c.c.  of  water,  and  0-25  gm.  of  ammonium  sulphate 
in  50  c.c.  of  water.  Add  5  c.c.  of  the  first  solution  to 
5  c.c.  of  the  second,  and  note  that  the  bulk  of  the 
precipitate  settles  in  a  few  minutes. 

Warm  the  ammonium  sulphate  solution  to  about 
30°  C.  and  add  to  it  5  c.c.  of  15  per  cent,  gelatin  sol, 
mix  thoroughly,  and  then  add  the  barium  chloride 
solution  with  continual  stirring.  The  precipitate 
does  not  settle  out  on  standing,  and  the  mixture 
passes  through  a  close  filter  paper  without  leaving 
any  residue.  Many  other  precipitates  may  be 
obtained  as  sols  in  this  way  by  adding  to  one 
solution  varying  amounts  of  gelatin,  albumin  or  gum 
arabic,  and  by  choosing  suitable  concentrations. 

The  other  procedure  is  to  add  to  a  sol,  for  which 
the  electrolyte  concentration  required  to  produce 
coagulation  in  a  definite  time  has  been  previously 
determined,  small  amounts  of  gelatin,  albumin  or 
gum  arabic  sol  and  to  determine  what  concentration 


GOLD  NUMBERS.  97 

of  electrolyte  will  now  produce  a  marked  change  or 
rapid  coagulation.  Thus,  9  c.c.  of  the  gold  sol  made 
by  the  formaldehyde  method  (p.  30)  turns  blue 
within  a  few  seconds  after  the  addition  of  i  c.c.  of 
N/i  NaCl  solution.  (Watch  this  change  by  pure 
transmitted  light,  say  by  looking  through  the  test 
tube  at  a  uniformly  illuminated  screen  of  white 
paper ;  reflected  light  must  be  excluded,  as  the 
strong  reddish-brown  surface  colour  of  the  sol  is 
almost  the  same  for  the  original  red  as  for  the  blue 
sol  and  makes  observation  of  the  change  difficult.) 
Now  add  to  9  c.c.  of  the  same  sol  i  c.c.  of  a  o-i  per 
cent,  gelatin  sol  (o-i  gm.  in  100  c.c.),  mix  well,  add 
i  c.c.  of  the  N/i  NaCl  solution,  and  note  that  no 
change  of  colour  occurs,  even  on  standing.  Add  more 
NaCl  solution,  i  c.c.  at  a  time,  and  note  that  even 
4  or  5  c.c.  produces  no  change  whatever. 

Gold  Numbers. — The  gold  number  of  an  emulsoid 
is  defined  by  R.  Zsigmondy  as  the  number  of  milli- 
grammes of  the  emulsoid  just  sufficient  to  prevent  a 
colour  change  in  10  c.c.  of  a  standard  red  gold  sol  on 
addition  of  i  c.c.  of  a  standard  solution  of  NaCl 
(density  1*07,  i.e.,  concentration  about  N/i).  As 
these  figures  are  arbitrary,  it  is  better  to  state  the 
percentage  concentration  of  emulsoid  in  the  gold  sol 
which  just  prevents  the  colour  change  when  i  c.c.  of 
N/i  NaCl  solution  is  added  to  about  10  c.c.  of  sol. 
Trie  following  figures  have  been  calculated  from 
Zsigmondy's  gold  numbers  : — 

Minimum  concentration  in 

Emulsoid.  per  cent,  which  prevents 

colour  change. 

Gelatin        .         .         .  0-00005  to  o-oooi 
Egg  albumin  (dry,  com- 
mercial)   .           .           .  O-OOI       tO  O-002 
Gum  arabic          .         .  0-0015    to  0-0025 
Dextrin       .         .         .  o-io       to  0-20 
Potato  starch       .         .  0-25 

L.M.  7 


98  GOLD  NUMBERS. 

These  are  exiguous  concentrations,  especially  for 
the  most  active  protective  agents  ;  if  i  c.c.  of  emul- 
soid  sol  is  to  be  added  to  10  c.c.  of  gold  sol,  the  con- 
centrations of  the  former  will  have  to  be  n  times 
those  given  in  the  table,  e.g.,  0-00055  to  o-oon  per 
cent.,  or  0-0055  to  o-on  gm.  per  litre,  for  gelatin.  It 
is  advisable  to  make  up  sols  of  still  greater  concen- 
tration, say  50  times  those  given  in  the  table, 
preferably  by  suitable  dilution  of  any  concentrated 
sols  available  in  the  laboratory. 

Then  proceed  as  follows  :  Place  10  c.c.  of  gold  sol, 
which  should  be  a  pure  red  without  any  purple 
tinge,  into  each  of  a  number  of  test  tubes,  add  o-l, 
0-2,  0-4  up  to  i  c.c.  of  emulsoid  sol  and  mix.  Then 
add  to  each  test  tube  i  c.c.  of  N/i  NaCl  and  note  the 
concentrations  in  the  tube  which  just  retains  the  red 
colour  and  the  next  one,  which  shows  the  change  to 
blue.  If,  for  instance,  the  tube  with  0-2  c.c.  of 
emulsoid  sol  remains  unaltered  and  the  one  with 
o-i  has  turned,  the  emulsoid  concentrations  are 
respectively  : — 

2/102  =0-0196  X  original  emulsoid  concentration, 
i/ioi  =0-0099  X         „  „  „ 

The  gold  number  in  percentage  lies  between  these 
values  and  may  be  determined  more  exactly  by  using 
intermediate  volumes  or  by  reducing  the  concen- 
tration of  the  emulsoid  sol  used  and  repeating  the 
series. 

The  gold  numbers  are  a  delicate  means  of  differen- 
tiating between  proteins  which  cannot  readily  be 
distinguished  by  other  tests.  The  method  has, 
therefore,  acquired  some  importance  in  medical  and 
biochemical  work,  which  is,  however,  beyond  the 
scope  of  this  book. 

Behaviour  of  different  Sols. — The  protective  effect 
of  a  given  emulsoid  sol  is  not  necessarily  the  same  on 
other  sols  as  on  gold  sols ;  apart  from  the  question 


SPECIFIC  DIFFERENCES.  99 

of  concentration  there  appear  to  be  specific  differ- 
ences, although,  generally  speaking,  the  various 
emulsoids  stand  in  the  same  order  for  most  sols  as 
they  do  for  gold  sols.  To  show  this,  determine 
approximtely  the  volume  of  N/i  NaCl  solution  neces- 
sary to  coagulate  10  c.c.  of  the  (dilute)  Prussian  blue 
sol.  Add  i  c.c.  of  the  0*1  per  cent,  gelatin  sol  pre- 
viously used  with  gold  sol  to  9  c.c.  of  Prussian  blue 
sol,  then  add  the  volume  of  sodium  chloride  solution 
found  sufficient  to  coagulate  the  unprotected  sol. 
Generally  this  will  be  sufficient  to  precipitate  the 
protected  sol  after  a  somewhat  longer  time,  not- 
withstanding the  presence  of  an  amount  of  gelatin 
which,  in  the  previous  experiment,  completely  pro- 
tected the  gold  sol. 

LITERATURE. 

For  protective  effect  and  gold  numbers,  see  Zsigmondy- 
Spear,  "  Colloidal  Chemistry "  (Chapman  and  Hall, 
1917),  pp.  106 — in  ;  "Investigations  on  a  Number  of 
Protective  Agents,  chiefly  of  Vegetable  Origin,"  by  A. 
Gut  bier  and  collaborators,  Koll.-Zeitschr.,  XVIII. ,  I, 
57,  141,  201 ;  XIX.,  22,  90,  177,  230  (1916) ;  XX., 
123,  186  (1917). 


CHAPTER  XV. 

VISCOSITY   MEASUREMENTS. 

THE  only  suitable  instrument  for  accurate  deter- 
minations is  a  properly  designed  capillary  viscometer. 
The  various  rough  methods  employed  occasionally, 
such  as  determining  the  time  required  for  a  given 
volume  to  flow  from  a  pipette,  or  the  time  taken  by 
a  small  sphere  to  fall  through  a  given  height,  are 
useless,  as  in  all  these  arrangements  the  time 
measured  is  very  far  from  being  simply  proportional 
to  the  viscosity. 

Two  types  of  capillary  viscometers  may  be  used  : 
the  simple  Ostwald  type  (Fig.  16),  in  which  the  flow 
is  caused  by  the  difference  of  head  in  the  two  limbs 
of  the  instrument,  and  the  Ubbelohde  type  (Fig.  17), 
in  which  the  flow  is  caused  by  a  constant  air  pressure 
applied  to  one  limb.  The  use  of  the  latter  and  the 
manostatic  apparatus  for  providing  constant  air 
pressure  will  be  described  later. 

The  Ostwald  viscometer  consists  of  a  wide  tube, 
generally  provided  with  a  bulb  at  the  lower  end, 
which  is  joined  by  a  bend  to  a  straight  capillary  tube. 
The  latter  leads  into  a  bulb  capable  of  holding  2  to 
3  c.c.  of  liquid,  and  provided  with  an  inlet  tube,  a 
constriction  being  provided  where  this  tube  joins  the 
bulb.  Marks  are  placed,  one  in  the  centre  of  the 
constriction  and  one  below  the  bulb  at  the  beginning 
of  the  straight  capillary.  The  instrument  is  gene- 
rally used  only  for  determining  relative  viscosities, 
e.g.,  the  viscosity  of  a  sol  referred  to  the  viscosity  of 
the  pure  dispersion  medium  as  unity.  In  this  case 
all  the  factors  in  Poiseuille's  formula  which  depend 


101 


on  the  measurements  of  the  instrument  remain  the 
same,  and  the  pressures  in  the  case  of  two  liquids 


B 


FIG.  16. 


FIG.  17. 


having  the  densities  p0  and  p:  are  simply  propor- 
tional to  these  densities,  provided  the  same  volume 
is  used,  so  that  the  effective  height  of  the  liquid  is 
the  same  in  both  cases.  If  the  times  taken  by  the 


102  OSTWALP  VISCOMETER. 

level  of  the  liquid  in  sinking  from  the  upper  to  the 
lower  mark  are  respectively  /0  and  tlf  the  viscosities 
YIQ  and  77  x  are  in  the  following  ratio  :  — 

*?q  :  *?l  ~ 
and  the  relative  viscosity 


It  is  accordingly  necessary  to  determine  the 
density  of  the  liquids  under  examination,  and  it  need 
hardly  be  added  that  this  must  be  done  at  the  tem- 
perature or  temperatures  at  which  the  viscosities  are 
to  be  measured. 

To  use  the  instrument  it  is  first  mounted  vertically, 
by  sighting  it  with  a  plumb-line  in  two  directions 
at  right  angles  to  each  other.  A  definite  volume  of 
liquid  is  then  run  into  the  wide  tube  from  a  pipette 
reserved  for  this  purpose.  A  convenient  length  of 
rubber  tube  is  fitted  to  the  top  of  the  bulb  and  the 
liquid  drawn  up  through  the  capillary  until  it  has 
filled  the  upper  bulb  and  risen  well  above  the  upper 
mark,  when  the  tube  is  closed  by  pinching  with  the 
left  hand.  (Sufficient  liquid  must  be  delivered  by 
the  pipette  to  leave  some  liquid  in  the  lower  bulb 
when  this  has  been  done.)  A  stop-watch  is  held  in 
the  right  hand,  the  rubber  tube  released  and  the 
watch  started  when  the  level  of  the  liquid  passes 
through  the  upper  mark,  and  stopped  when  it  passes 
through  the  lower  mark.  The  eye  should  be  kept  on 
the  viscometer  and  not  on  the  watch. 

As  viscometers  are  not  readily  obtainable,  they 
will  generally  have  to  be  blown  specially,  when  the 
following  points  are  to  be  noted.  For  sols  with 
water  as  dispersion  medium  and,  therefore,  requiring 
the  time  of  outflow  of  water  to  be  determined, 
capillaries  about  0-5  to  0-6  mm.  bore  will  be 
suitable.  The  length  of  the  capillary  should  be 
80  to  100  times  the  diameter  of  the  bore,  say  5  to 


OSTWALD  VISCOMETER.  103 

6  cm.  at  least.  A  bulb  holding  2  to  2-5  c.c.  on  the 
end  of  the  capillary  is  convenient,  and  the  diameter 
of  the  inlet  tube,  as  well  as  that  of  the  bend  connect- 
ing the  capillary  with  the  wide  tube,  should  not  be 
less  than  3  mm.,  preferably  4  mm.  The  change  in 
diameter  from  the  bulb  into  the  capillary,  and  from 
the  latter  into  the  bend,  should  be  smooth  and 
gradual.  The  time  of  flow  should  not  be  less  than 
60  seconds  for  water  at  20°  C. 

Viscometers  conforming  to  this  description  will  be 
suitable  for  measuring  viscosities  up  to  20  or  25 
times  that  of  water,  i.e.,  for  fairly  concentrated 
emulsoid  sols.  Sols  having  organic  dispersion  media, 
such  as  rubber-benzene  sols,  or  sols  of  nitrocellulose 
in  various  media,  however,  often  have  viscosities  of 
much  larger  order  even  in  moderate  concentrations, 
and  it  is  then  not  feasible  to  carry  out  the  whole 
series  of  measurements  with  the  instrument  used  for 
the  dispersion  medium.  A  range  of  viscometers  of 
increasing  bore,  and  proportionately  increasing 
length  of  capillary,  must  then  be  provided.  The 
readings  made  by  two  instruments,  say  one  used  for 
the  dispersion  medium  and  for  relative  viscosities 
up  to  20,  and  the  next  instrument,  are  then  connected 
with  each  other  in  the  following  manner.  The  time 
of  efflux  for  the  most  concentrated  sol,  for  which  the 
first  instrument  can  be  used,  is  determined  and 
found  to  be  wr  The  time  of  efflux  for  the  same  sol 
is  then  determined  in  the  second  instrument  and 
found  to  be  a  smaller  value,  «2.  To  reduce  readings 
on  the  second  instrument  to  those  on  the  first,  and 
eventually  to  relative  viscosities,  they  must,  there- 
fore, be  multiplied  with  n^/n^  Instead  of  the 
sol  itself  any  sufficiently  viscous  liquid  may  be  used 
to  determine  this  ratio  ;  glycerine  or  mixtures  of 
glycerine  with  (little)  water  may  be  employed. 

Viscometers  must  be  thoroughly  cleaned  imme- 
diately before  use  with  hot  dichromate-sulphuric 


104  THERMOSTAT. 

acid  mixture,  followed  by  distilled  water  and  then 
by  alcohol  and  ether,  which  is  dried  off  by  blowing 
air  filtered  through  glass  wool  through  the  instru- 
ment. The  latter  precaution  is  necessary,  as  even 
small  particles  of  dust  may  vitiate  results  seriously, 
in  view  of  the  small  bore  of  the  capillary.  For  the 
same  reason  the  liquids  to  be  investigated  should  be 
filtered,  or,  where  this  is  not  possible,  at  least 
strained  through  a  glass  wool  plug. 

Since  viscosity  decreases  to  the  extent  of  3  to 
5  per  cent,  per  degree  of  temperature  in  pure  liquids, 
and  at  a  much  higher  rate  in  emulsoid  sols,  it  is 
absolutely  essential  that  measurements  should  be 
carried  out  in  a  thermostat  in  which  it  is  possible  to 
keep  temperatures  constant  within  0-1°  C.  Where 
a  proper  apparatus  is  not  available  a  beaker  holding 
at  least  two  litres  may  be  used.  The  viscometer,  a 
thermometer  divided  into  tenths  of  a  degree,  and  the 
toluene  regulator  are  supported  in  the  water  from  a 
suitable  stand,  and  some  form  of  stirrer  must  be 
arranged.  The  beaker  stands  on  asbestos-coated 
wire  gauze  on  a  tripod,  and  is  heated  by  a  small — 
pin-hole  or  "  micro  " — gas  burner,  which  must, 
however,  be  sufficient  to  keep  the  water  at  the 
required  temperature,  i.e.,  to  make  up  the  heat  lost 
by  convection  and  radiation.  The  water  may  be 
heated  up  to  within  a  degree  of  the  required  tempera- 
ture by  a  Bunsen  burner,  and  the  small  burner  sub- 
stituted for  it  then.  The  regulator  is  then  adjusted 
so  as  to  cut  off  about  the  required  temperature  ; 
since  it  is  somewhat  troublesome  to  do  this  with 
complete  accuracy,  it  is  better  to  take  readings 
within  0*2  or  0-3°  of  round  numbers  rather  than 
to  spend  much  time  in  trying  to  set  the  regulator 
exactly  to  the  latter.  Thus,  if  measurements  at, 
say,  five  degrees'  intervals  are  wanted,  it  will  be  quite 
permissible  to  work  at  20-3°,  25-1°,  30-0°,  etc.,  pro- 
vided the  results  are  plotted  accurately. 


VISCOMETER— CONSTANT  PRESSURE.   105 

The  temperature  regulation  may  fail  in  two  ways  : 
either  the  temperature  may  fall,  although  the  regu- 
lator is  fully  open — in  that  case  the  gas  pressure  or  the 
size  of  the  burner  is  insufficient ;  or  the  gas  regulator 
may  fail  to  cut  off,  although  the  temperature  keeps 
rising — this  trouble  is  generally  due  to  "  creeping  " 
of  the  toluene,  which  passes  between  the  mercury 
and  the  glass,  instead  of  raising  the  mercury.  This 
can  be  corrected  by  renewing  the  toluene  and 
thoroughly  cleaning  the  mercury.  The  trouble  is 
much  less  frequent  if  a  concentrated  solution4  of 
calcium  chloride  in  water  is  substituted  for  the 
toluene. 

The  determination  of  the  density — unless  required 
for  some  further  reason — is  tedious  and  can  be 
avoided  by  the  use  of  the  viscometer  illustrated  in 
Fig.  17,  in  which  the  pressure  causing  the  flow  is 
produced  by  compressed  air  instead  of  by  the  head 
of  liquid  itself.  The  instrument  has  two  bulbs  of 
equal  size ;  one  limb  of  the  U  connecting  the  bulbs  is 
a  capillary  of  suitable  bore,  while  the  other  is  a  wider 
tube.  The  liquid  is  drawn  into  the  viscometer 
through  A,  and  a  definite  volume  must  be  used,  so 
that,  when  the  level  is  at  B  in  the  bulb  on  the 
capillary  side,  it  stands  at  C  in  the  opposite  limb. 
When  pressure  is  applied  on  the  side  containing  the 
capillary,  the  liquid  rises  into  the  opposite  bulb,  and, 
finally,  the  difference  of  levels  is  equal  and  opposite 
to  that  which  prevailed  at  the  beginning,  so  that  the 
effect  of  the  liquid  head,  and,  therefore,  of  the 
density,  is  eliminated. 

The  pressure  is  generated  by  the  simple  manostat 
shown  in  Fig.  18.  A  Mariotte's  bottle  A  discharges 
water  through  a  rubber  tube  and  a  piece  of  glass  tube 
turned  upwards  at  a  right  angle  into  a  second  bottle 
B  of  the  same  size.  A  Tee-piece  passing  through  the 
stopper  in  the  top  of  the  bottle  is  connected  at  C  to 
a  water  pressure  gauge  and  at  D  to  the  viscometer, 


io6  VISCOMETER— CONSTANT  PRESSURE. 

which  is  fitted  with  a  three-way  stop-cock.  The 
active  column  of  water  is  that  between  the  bottom 
of  the  air  tube  of  the  Mariotte  bottle  and  the  top  of 


FIG.  18. 


the  bent  tube  discharging  the  water,  and  the  gauge 
must,  of  course,  have  a  limb  of  somewhat  greater 
length  than  this  height. 

Before  using  the  apparatus,  water  is  allowed  to 
flow  into  the  lower  bottle  until  the  column  in  the 


VISCOMETER— CONSTANT  PRESSURE.   107 

gauge  becomes  stationary.  While  this  is  being  done 
the  three-way  cock  is  turned  so  as  to  shut  off  the 
compressed  air  tube  from  the  viscometer  and  to 
leave  the  latter  open  to  the  atmosphere.  The 
viscometer  is  now  filled  with  the  required  volume  of 
liquid,  which  is  drawn  up  into  the  left-hand  limb 
above  the  bulb  by  suction  applied  at  E.  The  cock 
is  then  turned  so  as  to  shut  off  the  viscometer  from 
the  atmosphere  and  leave  the  compressed  air  supply 
also  shut  off.  The  stop-watch  is  now  got  ready,  the 
compressed  air  admitted  to  the  viscometer,  and  the 
watch  is  released  as  the  liquid  passes  through  the 
lower  mark  and  arrested  when  it  passes  through  the 
upper  mark.  All  measurements  in  one  series  are, 
of  course,  carried  out  with  the  same  air  pressure, 
i.e.,  without  altering  the  column  of  the  manostat. 
The  time  between  marks  is  then  directly  proportional  to 
the  viscosity. 

It  is  hardly  necessary  to  add  that  this  type  of 
instrument,  like  the  Ostwald  viscometer,  must  be 
kept  in  a  thermostat.  In  both  cases  it  is  essential 
to  make  sure  that  the  liquid  under  examination  has 
reached  the  temperature  indicated  by  the  thermo- 
meter in  the  water  bath  ;  this  is  the  case  if  two 
readings  taken  at  an  interval  of,  say,  five  minutes  do 
not  differ  by  more  than  i  per  cent,  at  the  outside. 
Generally  speaking,  three  determinations  on  the 
same  specimen  should  always  be  made  and  the 
arithmetical  mean  taken  as  the  final  result,  provided 
the  three  readings  do  not  differ  by  more  than  i  per 
cent.  If  the  readings  decrease,  the  temperature 
may  still  be  rising,  as  already  pointed  out ;  if,  how- 
ever, the  decrease  becomes  more  marked  on  repeti- 
tion, the  viscosity  of  the  sol  is  being  reduced  through 
its  being  forced  through  the  capillary.  This  pheno- 
menon is  quite  common  with  many  emulsoid  sols 
and  very  marked,  e.g.,  with  starch  sols. 

If  the  readings  increase  on  repetition  (provided,  of 


io8          PLOTTING  MEASUREMENTS. 

course,  that  the  thermostat  is  working  properly),  the 
capillary  is  becoming  blocked,  either  by  accidental 
contamination  with  dust,  etc.,  or  by  adsorption  or 
coagulation  on  its  wall.  In  that  case  the  instrument 
must  be  removed  and  thoroughly  cleaned  in  the 
manner  already  described. 

In  general  series  of  viscosity  measurements  will  be 
carried  out  to  determine,  e.g.,  the  change  in  viscosity 
with  concentration,  or — concentration  being  con- 
stant— with  temperature.  In  both  cases  the  results 
should  be  plotted  on  sectional  paper,  to  a  fairly  large 
scale,  with  the  variable  concentration  (or  tempera- 
ture) as  abscissa  and  the  viscosity  as  ordinate.  The 
latter  is,  of  course,  proportional  to  the  product: 
time  x  density,  if  the  Ostwald  viscometer  is  used, 
and  to  the  time  only  with  the  Ubbelohde  viscometer. 
The  points  found  should,  in  general,  lie  very  nearly 
on  a  smooth  curve  ;  cusps  or  inflexions  occur  only 
where  coagulation  or  the  like  takes  place.  Points 
which  fall  outside  a  smooth  hyperbolic  or  logarithmic 
curve  to  a  marked  extent  are,  therefore,  suspect 
unless  such  disturbing  phenomena  are  at  all  pro- 
bable, and  the  particular  reading  should  be  carefully 
repeated. 

To  practise  the  use  of  the  apparatus  the  beginner 
will  find  gum  arabic  sol  convenient.  Five  concen- 
trations, say  5,  10,  15,  20  and  30  per  cent.,  should  be 
prepared,  filtered  through  glass  wool  and  the  vis- 
cosities determined  at  some  convenient  temperature, 
say  2°  or  3°  C.  above  that  of  the  room.  The  sols 
should  be  prepared  when  required.  For  viscosity- 
temperature  measurements  a  sol  made  from  30  gm. 
of  gelatin  in  100  c.c.  of  water  is  convenient,  made 
and  filtered  in  the  usual  way.  Measurements  should 
be  begun  with  the  thermostat  at,  say,  45°  or  50° ; 
the  temperature  is  then  allowed  to  fall  about  5°,  the 
regulator  readjusted  and  a  reading  taken,  and  this 
procedure  is  continued  until  the  setting  temperature 


TEMPERATURE—  VISCOSITY   CURVES.    109 

of  the  sol  is  reached.  The  results  may  be  plotted 
directly,  i.e.,  as  the  products  time  X  density  ;  a 
clearer  insight  is,  however,  gained  by  plotting 
relative  viscosities  with  water  at  the  same  temperature 
taken  as  unity.  The  viscosity  of  water  at  different 
temperatures  may  be  found  in  tables,  or  be  deter- 
mined in  a  second  viscometer  placed  in  the  thermo- 
stat ;  the  densities  of  water  at  different  tempera- 
tures are  given  in  most  works  of  reference.  The 
quantities  to  be  plotted  will  then  be,  if  the  times  of 
efflux  and  the  densities  of  the  sol  at  the  temperatures 
i,  2,  etc.,  are  tl  and  plt  t2  and  />2,  etc.,  and  the 
corresponding  figures  for  water  tf±  and  p'lt  t'2  and  p'2. 


A  curve  plotted  with  these  relative  viscosities  as 
ordinates  against  temperatures  as  abscissae  shows 
that  the  temperature  coefficient  of  the  sol  is  much 
greater  than  that  of  water,  i.e.,  the  percentage 
decrease  with  rising  temperature  is  much  greater 
than  that  of  pure  water. 

LITERATURE. 

See,  generally,  Faraday  Soc.  Gen.  Discussion  on  Colloids 
and  their  Viscosity,  1913  ;  copious  references  to  experi- 
mental work  are  given  in  this,  especially  in  Wo.  Ostwald's 
contribution. 


CHAPTER  XVI. 

ADSORPTION   (QUALITATIVE 
EXPERIMENTS). 

THE  removal  of  solutes  from  solution  by  solids 
having  a  large  surface  can  be  shown  in  a  great  variety 
of  ways.  Solutions  of  dyes,  e.g.,  crystal  violet, 
methyl  violet,  methyl  green,  etc.,  containing  2  to 

3  mg.  in  100  c.c.,  may  be  shaken  with  2  to  3  gm. 
of  charcoal,  fuller's  earth   or  china   clay,  and  will 
generally  be  found  colourless  after  the  adsorbent 
has  settled.     The  adsorption  of  lead  salts  is  another 
striking  example.     Add  to  100  c.c.  of  water  2  or  3 
drops  of  concentrated  solution  of  lead  nitrate,  take 
5  c.c.  of  the  mixture  and  note  the  reaction  with 
ammonium  sulphide.     Then  shake  the  bulk  with 

4  to  5  gm.  of  charcoal,  filter  and  test  the  filtrate  with 
ammonium  sulphide  ;   with  moderately  good  brands 
of  charcoal  no  reaction,  or  at  most  a  very  faint 
brown  tinge,  will  be  visible. 

Influence  of  Solvent  on  Adsorption. — Dissolve  2  to 
3  mg.  of  methyl  violet  in  100  c.c.  of  water,  shake 
with  2  gm.  of  charcoal  and  allow  the  latter  to 
settle  ;  the  supernatant  liquid  is  generally  colour- 
less. Pour  it  off  as  far  as  possible,  and  pour  on  the 
charcoal  80  to  90  c.c.  of  alcohol  or  acetone.  This 
immediately  assumes  a  violet  colour,  showing  that 
the  equilibrium  concentration  in  the  organic  solvent 
is  higher  than  in  water,  i.e.,  the  amount  adsorbed 
from  it  is  smaller. 

Adsorption  due  to  Neutralization  of  Electric  Charges. 
— A  glass  tube,  about  25  mm.  diameter  and  about 


ELECTRIC  &  SELECTIVE  ADSORPTION,   in 

60  cm.  long,  is  held  vertically  in  a  suitable  clamp. 
The  lower  end  is  closed  by  a  rubber  stopper,  through 
which  passes  a  short  piece  of  glass  tube  about  4  mm. 
diameter,  the  upper  end  of  which  is  flush  with  the 
surface  of  the  stopper,  while  the  lower  projects 
i  or  2  cm.  Place  a  loose  plug  of  glass  wool  on  the 
stopper,  and  then  fill  the  tube  to  half  its  height  with 
silver  sand,  which  has  been  washed  with  nitric  acid 
followed  by  water,  and  has  then  been  dried  and 
ignited.  Then  fill  the  tube  with  ferric  hydroxide  sol 
and  collect  the  liquid  which  escapes  from  the  outlet 
tube  in  a  beaker.  This  is  quite  colourless,  the  ferric 
hydroxide,  which  is  positive,  having  been  discharged 
and  retained  by  the  negatively  charged  quartz 
grains.  If  Night  blue,  a  dye  which  is  also  positively 
charged  in  aqueous  dispersion,  can  be  obtained,  the 
sol  may  be  used  instead  of  ferric  hydroxide  ;  2  to 
4  mg.  in  100  c.c.  is  a  suitable  concentration. 

Similar  results  may  be  obtained  by  allowing  strips 
of  filter  paper  (which  also  takes  a  negative  charge 
in  water)  to  dip  into  sols.  If  the  latter  are  positive, 
only  water  rises,  the  disperse  phase  being  coagulated 
at  the  level  of  the  liquid  ;  if  the  sol  is  negative,  no 
separation  occurs  and  the  colour,  e.g.,  of  Prussian 
blue,  rises  in  the  strip. 

Selective  Adsorption. — An  example  can  be  demon- 
strated as  follows  :  Dissolve  5  gm.  of  gelatin  in 
50  c.c.  of  water  in  the  usual  manner,  and  pour  the 
sol  into  shallow  moulds,  so  as  to  obtain  strips  or 
discs  3  to  4  mm.  thick.  Remove  these  from  the 
moulds  after  12  hours,  and  place  them  in  a  flat 
porcelain  (developing)  dish  containing  about  150  c.c. 
of  a  2  per  cent,  solution  of  commercial  aluminium 
sulphate.  Place  10  c.c.  of  the  solution  in  a  test  tube, 
add  a  few  drops  of  ammonium  thiocyanate,  note 
that  the  solution  shows  a  marked  iron  reaction,  and 
set  the  sample  aside.  After  lying  in  the  solution 
for  three  or  four  days  the  gelatin  shows  a  marked 


ii2  ADSORPTION. 

reddish-brown  tinge  due  to  ferric  iron  ;  if  a  10  c.c. 
sample  of  the  solution,  in  which  the  gelatin  is  lying, 
is  again  tested  with  thiocyanate  and  compared  with 
the  original  sample,  the  iron  content  will  be  found 
to  be  much  reduced. 


CHAPTER  XVII. 

CAPILLARY  ANALYSIS. 

THIS  is  a  method  of  separating  and  detecting 
various  constituents  of  a  mixture  by  means  of  the 
difference  in  their  rates  of  diffusion  and  adsorption. 
The  usual  procedure  is  to  allow  the  solution  con- 
taining the  several  solutes  to  rise  in  strips  of  white 
filter  paper  ;  the  various  constituents  rise  to  different 
heights,  and  may  be  detected  by  their  colour,  or  by 
suitable  reagents  applied  to  different  portions  of 
the  strip. 

The  strips  should  be  cut  from  a  white,  neutral 
filter  paper  (Whatman  No.  2  is  suitable)  about 
i  cm.  wide  and  25  to  30  cm.  long  ;  the  edge  of  the 
sheet  must  not  be  used.  They  are  then  suspended 
vertically,  with  their  lower  ends  dipping  about  2  cm. 
into  the  liquid  to  be  examined.  Evaporation  must 
be  prevented  ;  for  single  experiments  the  simplest 
way  is  to  place  the  liquid  in  the  bottom  of  a  tall 
cylinder  and  suspend  the  strip  from  the  stopper,  care 
being  taken  that  it  hangs  vertically  and  does  not 
touch  the  wall.  The  liquid  is  allowed  to  rise  until 
it  becomes  stationary  or  for  a  fixed  time,  say  6 
to  9  hours,  and  the  strip  is  then  examined  and 
tested. 

The  following  is  a  convenient  example  for  showing 
the  delicacy  of  the  method.  Slice  about  100  gm. 
of  boiled  beetroot,  pulp  the  slices  with  50  c.c.  of 
5  per  cent,  acetic  acid,  place  the  pulp  into  a  muslin 
bag  and  express  about  50  c.c.  of  liquid,  which  need 
not  be  filtered.  Take  5  c.c.  of  the  liquid  and  add  to 


H4  CAPILLARY  ANALYSIS. 

it  gradually  in  a  test  tube'N/25  caustic  soda  solution  ; 
the  colour  changes  to  purple,  brown  and,  finally,  to 
a  dirty  greenish  yellow.  Now  add  to  the  45  c.c.  of 
liquid  three  to  five  (burette)  drops  of  the  methyl 
orange  used  as  indicator.  This  turns  red,  the  colour 
being  entirely  masked  by  the  deep  red  of  the  solu- 
tion ;  the  colour  change  with  alkali  is  similarly 
masked,  as  the  beetroot  pigment  also  turns  yellow. 
Place  the  liquid  to  which  methyl  orange  has  been 
added  into  a  tall  cylinder  and  suspend  a  strip  of 
filter  paper  as  previously  described.  The  strip  is 
gradually  stained  a  fairly  uniform  purplish  red. 
When  this  has  reached  a  height  of  about  16  or  18  cm. 
remove  the  strip,  let  it  drain  for  a  few  minutes  and 
then  dip  it  into  N/25  solution  of  NaOH,  removing  it 
immediately.  The  clear  yellow  of  the  methyl  orange 
turned  by  alkali  is  very  plainly  visible  at  the  top, 
over  a  width  of  2  or  3  cm.,  while  the  rest  of  the  strip 
still  remains  purple  or  red.  If  the  strip  is  imme- 
diately rinsed,  dried  and  kept  in  the  dark,  the  result 
may  be  preserved  permanently ;  failing  this,  the 
lower  portion  gradually  gets  discoloured. 

Another  mixture  suitable  for  demonstrating  the 
method  is  made  by  extracting  5  gm.  of  turmeric  with 
30  to  40  c.c.  of  hot  water,  filtering  and  adding  to  the 
filtrate  about  i  c.c.  of  concentrated  picric  acid.  The 
mixture  stains  the  filter  paper  a  fairly  uniform 
yellow ;  when  dipped  in  dilute  caustic  soda,  the 
lower  portion  turns  brown,  while  the  upper,  which 
contains  the  acid  only,  remains  yellow  with  sodium 
picrate. 

The  method  is  capable  of  very  wide  application. 
It  has  been  developed,  and  its  possibilities  demon- 
strated, chiefly  by  F.  Goppelsroeder,  whose  work 
unfortunately  has  appeared  chiefly  in  publications 
not  generally  accessible.  A  number  of  papers 
covering  a  very  wide  field  were  published  in  the 
Kolloid-Zeitschriff,  and  are  given  below. 


CAPILLARY  ANALYSIS.  115 

LITERATURE. 

F.  Goppelsroeder,  Koll.-Zeitschr.,  V.,  52  (salt  solu- 
tions), 109  (foods  and  beverages),  159  (mineral  waters), 
200  (vegetable  pigments),  305  (urine)  (1909)  ;  VI.,  42 
(urine  continued),  in  (vital  staining  of  plants),  174 
(vital  staining  of  animals),  213  (vital  staining  of  animals 
continued),  268  (constitution  of  dyes  and  vital  staining) 
(1910). 


1-8 


CHAPTER  XVIII. 

DETERMINATION  OF  AN  ADSORPTION 
ISOTHERM. 

A  COMPARATIVELY  simple  and  satisfactory  instance 
is  the  adsorption  of  oxalic  acid  by  charcoal,  the  acid 
concentration  being  determined  by  titration  with 
potassium  permanganate. 

Dissolve  10-5  gm.  of  crystallized  oxalic  acid 
(C2H2O4  .  2H2O)  to  make  250  c.c.  of  solution.  Then 
place  into  five  conical  beakers  or  Erlenmeyer  flasks 
of  100  to  150  c.c.  capacity  the  following  :  50  c.c.  of 
the  original  acid  solution ;  40  c.c.  of  solution  and 
10  c.c.  of  water ;  30  c.c.  of  solution  and  20  c.c.  of 
water  ;  20  c.c.  of  solution  and  30  c.c.  of  water,  and 
10  c.c.  of  solution  and  40  c.c.  of  water.  Label  the 
beakers  in  the  same  order,  5,  4,  3,  2  and  i  respec- 
tively, these  being  the  ratios  of  their  original  con- 
centrations. 

Place  in  each  beaker  i  gm.  of  finely  powdered 
charcoal  and  shake  well  at  intervals,  and  finally  allow 
the  powder  to  settle  overnight. 

We  have  now  to  consider  the  choice  of  a  suitable 
strength  for  the  permanganate  solution.  The  solu- 
tion (i)  has  a  concentration  before  adsorption  one- 
fifth  of  that  of  the  initial  solution,  and  this  will  pre- 
sumably be  considerably  reduced.  The  final  con- 
centration after  adsorption  should,  however,  still  be 
capable  of  accurate  determination,  and  we  shall, 
therefore,  do  well  to  choose  a  permanganate  solution 
so  dilute  that,  say,  25  c.c.  of  it. will  be  required  to 
oxidize  5  c.c.  of  the  starting  solution. 


ADSORPTION  ISOTHERM.  117 

The  concentration*  of  the  latter  (C2H2O4  .  2H20 
=  126)  is  M/3,  and  we  shall,  therefore,  require 
2KMnO4/i5  for  complete  oxidation  of  one  litre  of 
solution,  i.e.,  21-086  gm.  Since,  however,  we  have 
settled  that  5  c.c.  of  permanganate  solution  should 
equal  i  c.c.  of  the  original  acid  solution,  we  shall 
require  one-fifth  of  that  concentration,  i.e.,  4-217  gm. 
per  litre.  It  will  not  be  necessary  to  make  up  a 
litre,  but  500  c.c.  (containing  2-108  gm.)  should  be 
made  up  with  freshly  distilled  water.  This  leaves  an 
ample  reserve  for  repeating  any  of  the  experiments 
which,  when  the  isotherm  comes  to  be  plotted, 
appear  doubtful ;  a  reserve  of  oxalic  acid  is  also 
provided  for  by  the  figures  given  above. 

The  first  operation  is,  of  course,  to  determine  the 
actual  ratio  of  permanganate  solution  to  oxalic  acid. 
Titrate  5  c.c.  of  the  stock  solution  with  the  perman- 
ganate solution  in  the  usual  way,  hot  in  presence  of 
sulphuric  acid.  Suppose  the  figure  found  is  26-1  c.c. 
(instead  of  25).  Then  5  c.c.  of  the  solutions  (4),  (3), 
(2)  and  (i)  will  respectively  require  20-88,  15-66, 
10-44  and  5 "22  c.c.  of  permanganate. 

Five  c.c.  of  each  of  the  solutions  is  now  pipetted 
off,  without  disturbing  the  sediment  of  charcoal,  and 
titrated.  Begin  with  (5),  remember  the  original 
titre,  26-1  c.c.,  bear  in  mind  that  the  concentration  will 
be  perceptibly  reduced  and,  therefore,  go  slowly.  If,  as 
will  quite  possibly  happen  in  one  case  or  another, 
the  end  point  is  overrun,  the  determination  must, 
of  course,  be  repeated.  Assuming  that  the  sample  (5) 
after  adsorption  requires  17-2  c.c.  only,  acid  equiva- 
lent to  26-1  —  17-2  =  8-9  c.c.  has  disappeared  from 
solution  by  adsorption.  In  proceeding  to  the  other 
samples,  remember  that  the  amounts  which  have 
been  adsorbed  will  be  smaller  absolutely  but  greater 
relatively.  As  charcoal  varies  very  considerably, 

*  In  this  chapter  "M"  is  used  to  denote  a  concentration  of 
one  mole  (gramme-molecule)  per  litre. 


n8  ADSORPTION  ISOTHERM. 

no  definite  figures  can  be  given  for  guidance.  The 
results  of  a  series  of  determinations,  made  exactly  as 
described,  are,  however,  given  below. 

Five  c.c.'  of  original  acid  solution  requires  26-5  c.c. 
of  KMn04. 

(5)         (4)  (3)  (2)  (i) 

5  c.c.  of  solution  requires 
c.c.  of  KMnO*  before  ad- 
sorption .  .  .  26-5  21-2  15-9  10-6  5-3 

5  c.c.  of  solution  requires 
c.c.  of  KMnO4  after  ad- 
sorption .  .  .  17-5  12-5  7-8  3-8  0-8 

Difference,  i.e.,  amount  ad- 
sorbed, in  c.c.  of  KMnO4.  9-0  8-7  8-1  6-8  4-5 

Since  all  our  units  are  arbitrary,  we  can  write  the 
usual  adsorption  formula  in  the  simple  form  : 

y  =  0C" 

where  y  is  the  amount  adsorbed  and  C  the  equili- 
brium concentration.  The  latter,  expressed  in  cubic 
centimetres  of  permanganate  solution,  is  given  by 
the  figures  in  the  second  row,  while  the  figures  in  the 
third  row  give  the  y  in  the  same  units.  We  can, 
therefore,  plot  the  C  as  abscissae  and  the  y  as  ordinates 
on  sectional  paper  to  a  convenient  scale,  say  i  c.c.  = 
I  cm.  The  points  so  obtained  should  lie  on  a  smooth 
curve  of  the  familiar  parabolic  type  (Fig.  19).  If 
any  points  fail  to  do  so,  the  corresponding  deter- 
mination should  be  immediately  and  carefully 
repeated. 

Although  the  curve  obtained  may  be  smooth  and 
have  the  general  appearance  of  the  adsorption 
isotherm,  it  is  not  possible  to  say  definitely  that  it 
conforms  to  the  equation  without  a  further  test. 
If  we  take  the  logarithms  on  both  sides,  we  find  : 

log  y  =  i/n  log  C  -f  log  a, 

which,  taking  log  y  and  log  C  as.  co-ordinates,  is  the 
equation  of  a  straight  line.  To  test  the  nature  of  the 
curve  we  must,  therefore,  plot  the  logarithms  of 


y— C  DIAGRAM. 


119 


y  and  C  as  ordinates  and  abscissae  respectively  ;  this 
can  be  done  by  plotting  the  actual  figures  on  logarithmi- 
cally ruled  paper,  or,  if  this  is  not  available,  by  taking 
the  logarithms  and  plotting  them  to  a  convenient 
scale,  say  o-i  =  I  cm.,  on  ordinary  millimetre  paper. 
The  logarithms  in  that  case  should  be  taken  to  three 


FIG.  19. 

figures,  with  the  last  figure  corrected.  We  thus 
obtain  the  following  values  for  the  results  found 
above  : 

(5)     (4)    (3)    (2)    (i) 
log  C  .  1-243   1-097  0-892  0-580 
log  y   .  0-954  0-940  0-908   0-833 

These  values  have  been  plotted,  lo 
and  log  y  as  ordinates,  in  Fig.  20,  an 
on  a  straight  line.  The  deviation  is 
appears  in  most  of  the  log  y — log 
found  in  the  literature.  Whether  it 
mental  error  or  actually  denotes  a 


0-903-1  =  -0-097 
0-653 

g  C  as  abscissae 
d  lie  very  nearly 
not  greater  than 

C  curves  to  be 
is  due  to  experi- 

departure  from 


120 


LOG  y— LOG  C  DIAGRAM. 


the  ideal  type  of  isotherm  can  only  be  determined 
by  further  test,  i.e.,  by  repetition  of  the  titrations 
of  (2)  and  (i),  and  by  determining  two  further  points, 
say  one  intermediate  and  one  below  (i).  Suitable 
mixtures  would  be  15  c.c.  of  the  original  acid  solution 
with  35  c.c.  of  water,  and  5  c.c.  of  the  original  acid 
solution  with  45  c.c.  of  water.  As  the  acid  in  the 
latter  will  be  almost  completely  removed,  it  may  be 


1.0 


0.9 


logC 
FIG.  20. 


1.0 


advisable  to  carry  out  the  titration  with  10  or  15 
(instead  of  5)  c.c.,  the  result  being  reduced  to  5  c.c. 
by  calculation.  In  view  of  the  smoothness  of  the 
y — C  curve,  the  deviation  from  the  straight  line, 
and,  therefore,  from  the  ideal  isotherm,  is  probably 
real. 

The  log  y — log  C  diagram  may  be  used  for  deter- 
mining the  value  of  n  in  the  equation  of  the  isotherm, 
since  n  =  log  C/(log  y  —  log  a)  is  the  cotangent  of 
the  angle  made  by  the  straight  line  with  the  C-axis. 


COMPARISON  OF  ISOTHERMS.        121 

Calculated  from  the  straight  line  joining  the  points 
(2),  (3),  (4)  and  (5),  this  in  the  present  case  would 
be  5-5,  so  that  i/n  —  0-18,  which,  although  low, 
comes  well  within  the  range  of  observed  values  of 
i/n.  Log  a  is,  of  course,  the  value  assumed  by  log  y 
when  log  C  =  o,  i.e.,  the  length  cut  off  by  the  straight 
line  on  the  y-axis ;  in  the  diagram  given  log  a  is 
about  0-68. 

Although  the  adsorption  isotherm  and  the  log  y — 
log  C  curve  may  be  plotted  with  arbitrary  units  as 
co-ordinates,  for  any  given  solute,  comparison  with 
another  substance  is  only  possible  if  the  results  are 
expressed  as  molar  concentrations.  This,  however, 
is  simply  a  matter  of  calculation.  Our  original  acid 
solution  contained  M/3  of  oxalic  acid.  Solution  (3), 
e.g.,  therefore  contained  M/3  x  3/5  =  M/5.  After 
adsorption  5  c.c.  of  (3)  required  7-8  c.c.  of  perman- 
ganate solution.  Since  5  c.c.  of  the  original,  M/3 
solution,  required  26-5  c.c.,  we  obtain  the  molar 
concentration  of  (3)  after  adsorption,  x,  from  the 
proportion  : 

M/3  :  x  =  26-5  :  7-8 
~x  —  M/io-2. 

We  therefore  know  that  i  gm.  of  the  charcoal  used, 
placed  in  50  c.c.  of  M/5  solution  of  oxalic  acid,  leaves 
an  equilibrium  concentration  of  M/io -2 .  This  enables 
us  to  compare  oxalic  acid  with,  say,  another  organic 
acid,  using,  of  course,  the  same  quantities  and  concen- 
trations, i.e.,  i  gm.  of  the  same  charcoal  and  50  c.c. 
of  M/5  solution  of  the  other  acid. 

The  example  discussed  has  been  chosen  as  being 
particularly  simple  for  two  reasons  :  the  solute  can 
be  used  in  fairly  high  concentrations,  and  the  method 
of  titration  is  a  very  accurate  one.  Similar  condi- 
tions, if  not  quite  so  favourable,  obtain  with  other 
organic  acids,  the  concentrations  being  determined 
by  ordinary  acidimetric  methods.  Adsorption  from 
mixtures  can  be  studied  when  a  specific  method  is 


122  SOURCES  OF  ERROR. 

available  for  titrating  one  constituent ;  thus  adsorp- 
tion from  mixtures  of  oxalic  and  some  other  acid  can 
be  investigated  by  determining  the  whole  acid  content 
acidimetrically,  and  the  concentration  of  oxalic  acid, 
in  a  parallel  sample,  by  permanganate. 

In  most  cases  the  difficulties  are,  however,  con- 
siderably greater,  and  resolve  themselves  chiefly  into 
finding  analytical  methods  of  sufficient  delicacy  to 
determine  small  differences  of  small  concentrations. 
Thus  with  many  dyes  the  whole  range  of  concentra- 
tions investigated  may  be  much  below  o-i  per  cent., 
while  no  specific  method  of  titration  is  available. 
Determinations  of  this  kind  have  been  made  by 
colorimetric  methods.  If  the  solute  is  optically 
active,  concentrations  may  be  determined  by  the 
polariscope,  provided,  of  course,  that  the  specific 
rotation  does  not  vary  with  the  concentration,  a  point 
which  must  be  ascertained  by  experiment  with  a  few 
solutions  of  known  strength  and  approximately  covering 
the  range  to  be  investigated. 

In  determining  the  adsorption  curve  the  assump- 
tion is  made  that  an  equilibrium  has  been  reached. 
Although  this  is,  roughly  speaking,  true  in  many 
cases,  numerous  instances  are  known  in  which  small 
amounts  of  solute  continue  to  disappear  from  the 
solution.  The  effect  of  this  continued  sorption  may 
show  itself  even  in  the  time  which  necessarily  elapses 
between  the  first  and  the  last  titration,  i.e.,  the 
values  found  for  the  samples  last  examined  are  some- 
what higher  relatively  than  those  for  the  first,  a 
discrepancy  which  would  show  itself  particularly  in 
the  log  y — log  C  curve.  If  there  is  reason  to  suspect 
this  phenomenon,  the  liquid  should  be  left  on  the 
adsorbent  and  determinations  repeated  at  intervals 
of  some  days.  The  causes  may  be  various,  e.g.,  the 
adsorption  on  the  coarse  external  surface  of  the 
adsorbent  is  followed  by  slow  diffusion  into  the  pores 
with  further  adsorption  on  the  surface  of  the  latter  ; 


CHOICE  OF  ADSORBENT.  123 

or  chemical  action  may  follow  adsorption,  a  possi- 
bility which,  although  apparently  remote,  has  been 
proved  real  in  some  instances ;  or,  finally,  the 
physical  condition  of  the  adsorbent,  and,  therefore, 
its  specific  surface,  may  change. 

For  theoretical  work  finely  powdered  charcoal, 
especially  blood  charcoal,  is  the  most  satisfactory 
adsorbent ;  fuller's  earth,  china  clay,  etc.,  give 
discrepant  results  more  frequently.  Whatever  adsor- 
bent is  selected,  an  amply  sufficient  quantity  to 
carry  out  and  repeat  experiments  should  be  obtained 
before  starting. 

LITERATURE. 

For  adsorption  by  solids  from  solution,  see  H.  Freund- 
lich,  "  Kapillarchemie,"  Leipzig,  1909,  pp.  145 — 173. 
Recent  papers  :  G.  v.  Georgievics,  Monatsh.  f.  Chem., 
34,  733  (1913),  ads.  of  acids  by  wool ;  T.  Oryng,  Kott.- 
Zeitschr.,  XIV.,  14  (1913),  negative  ads.  ;  K.  Estrup, 
Koll.-Zeitschr.,  XIV.,  8  (1914),  ads.  of  electrolytes; 
Sorption  of  iodine  by  carbon,  J.  W.  McBain,  Trans. 
Faraday  Soc.,  XIV.,  Part  3  (1919);  C.  Koch,  Koll.- 
Zeitschr.,  XXII.,  i  (1918),  adsorption  of  sodium  auri- 
chloride  on  charcoal ;  a  good  example  of  adsorption  in 
very  low  concentrations. 


CHAPTER  XIX. 
THE  LIESEGANG  PHENOMENON. 

R.    E.    LIESEGANG'S   original   prescription   is   as 
follows  r  4  gm.  of  gelatin  is  dispersed  in  100  c.c.  of 
water  in  the  usual  way,  and  2  c.c.  of  a  concentrated 
solution  of  potassium  dichromate  added  to  the  sol. 
The  mixture  is  poured  on  clean  glass  plates  to  form 
a  thin  layer,  about  0-45  c.c.  per  square  inch  of  surface 
being  allowed.     The  plate  is  supported  on  a  hori- 
zontal surface  and  the  sol  allowed  to  set ;    10  to  15 
minutes  will  be  required,  according  to  the  temperature 
of  the  room.     A  large  drop  of  20  to  30  per  cent, 
solution  of  silver  nitrate  is  placed  in  the  centre  of  the 
plate,  preferably  by  allowing  five  successive  drops 
of  about  o-i  c.c.  each  to  fall  on  the  same  spot  from  a 
small  pipette  drawn  into  a  sufficiently  fine  point. 
If  this  operation  is  properly  carried  out,  the  drop 
should  have  a  clean  circular  outline.     The  plate  is 
kept  in  the  dark  for  24  to  48  hours  (as  light  acts  on 
gelatin  containing  dichromate),  but  may  be  examined 
from  time  to  time  in  diffuse  light.     At  the  end  of  this 
period  any  traces  of  the  original  drop  still  remaining 
may  be  removed  with  a  pointed  strip  of  filter  paper, 
and  the  gel  is  then  allowed  to   dry.     The  silver 
chromate  resulting  from  the  reaction  will  be  found 
to  form  numerous  concentric  circles  round  the  edge 
of  the  original  drop,  separated  by  clear  zones  free 
from  precipitate  and  increasing  in  width  from  the 
centre  outwards. 

The  following  details  should  be  noted.  The  plates 
must  be  quite  clean  and,  in  particular,  free  from 
traces  of  grease.  To  cover  them  with  gelatin  right 


SILVER  CHROMATE  RINGS.  125 

up  to  the  edge  is  an  operation  requiring  considerable 
practice,  and  the  beginner  may  be  satisfied  with  a 
uniform  layer  extending  to  within  £"  of  it.  The 
plate  should  be  slightly  warmed  and  held  in  the  left 
hand  an  inch  or  two  above  the  horizontal  surface  on 
which  the  plate  is  eventually  allowed  to  cool,  while 
the  whole  amount  of  sol  is  poured  slowly  on  the 
centre  and  uniformly  spread  by  slightly  inclining  the 
plate  as  may  be  necessary.  After  cooling  and  before 
putting  on  the  silver  nitrate  the  plate  should  be  placed 
where  it  can  be  left  undisturbed  for  the  rest  of  the 
time,  as  the  drop  easily  spreads  if  the  plate  is  moved. 

To  produce  really  good  rings  the  gelatin  must 
contain  a  small  amount  of  acid  and  of  gelatose  (a 
product  of  hydrolysis  which  does  not  gelatinize  on 
cooling).  Inferior  commercial  brands  of  gelatin 
happen  to  contain  these  two  constituents  in  the  right 
proportion,  while  particularly  "  hard  "  gelatins  may 
require  a  slight  addition  of  either  or  both.  Liesegang 
recommends  citric  acid  as  particularly  suitable,  and 
the  addition  of  5  to  10  drops  of  a  5  per  cent,  solution 
to  100  c.c.  of  sol  may  be  tried  if  a  particular  brand 
of  gelatin  does  not  give  good  rings.  Similar  quantities 
of  gelatose  may  also  produce  marked  improvement ; 
it  may  be  prepared  by  prolonged  boiling  of  a  10  per 
cent,  gelatin  sol  (evaporated  water  being  replaced), 
which  is  continued  until  a  sample  placed  on  a  cold 
glass  surface  no  longer  sets  to  a  jelly.  Suitable  pro- 
portions of  either  or  both  constituents  increase  the 
width  of  the  chromate  rings  until,  with  excessive 
amounts,  the  whole  precipitate  forms  a  continuous 
band.  Instead  of  adding  gelatose  it  may  also  be 
produced  in  the  sol  itself  by  keeping  it  at  high  tem- 
perature for  several  hours  ;  the  dichromate  must, 
of  course,  not  be  added  until  this  operation  is  com- 
plete, as  it  would  undergo  partial  reduction. 

The  experiment  may  also  be  carried  out  in  a  some- 
what different  manner.  A  test  tube  is  filled  to  about 


126  VARIOUS  REACTIONS 

two-thirds  of  its  height  with  the  dichromate-gelatin 
sol,  which  is  allowed  to  set,  and  a  few  cubic  centi- 
metres of  the  silver  nitrate  solution  is  then  poured  on 
top  of  the  gel.  Other  reactions,  however,  give  better 
results  with  this  procedure,  among  which  the  follow- 
ing are  particularly  suitable  for  study  : — 

Tricalcium  Phosphate  in  Gelatin. — Dissolve  3  gm. 
of  crystallized  tribasic  sodium  phosphate  (Na3PO4  . 
I2H2O)  in  100  c.c.  of  distilled  water  and  pour  the 
solution  on  10  gm.  of  gelatin.  Allow  the  latter  to 
swell  for  three  to  four  hours,  then  disperse  on  the 
water  bath  at  100°  C.  and  filter  at  80°  to  90°.  Even 
the  best  brands  of  gelatin  give  a  precipitate  with  the 
phosphate,  but  the  procedure  prescribed  makes  it 
coarser  than  it  would  be  if  the  sodium  salt  were 
added  to  the  sol.  Test  tubes,  ¥  or  f "  diameter,  are 
filled  with  the  filtered  sol  to  about  two-thirds  and 
allowed  to  cool  slowly.  The  sol  must  be  poured 
slowly  down  the  side  of  the  test  tube,  to  avoid  the  for- 
mation of  froth  or  bubbles.  After  the  tubes  have 
stood  for  at  least  one  hour,  any  of  the  following 
solutions  may  be  poured  on,  all  of  which  give 
numerous  excellent  stratifications  :  10  per  cent. 
CaCl2 ;  a  mixture  of  two  parts  of  10  per  cent.  CaCl2 
and  three  parts  of  10  per  cent.  NaCl  solution  ;  20  per 
cent,  crystallized  calcium  nitrate  (Ca(N03)2  .  4H2O). 
Formation  of  strata  continues  down  to  the  bottom 
of  the  tube  and  is  complete  in  seven  to  ten  days. 

Lead  Iodide  in  Agar. — Dissolve  4  gm.  of  potassium 
iodide  in  100  c.c.  of  i  per  cent,  agar  sol,  prepared  and 
filtered  as  described  above.  Pour  the  sol  into  test 
tubes  exactly  as  explained  in  the  preceding  para- 
graph and  allow  them  to  cool  slowly.  When  they 
have  reached  the  room  temperature,  pour  on  a 
30  per  cent,  solution  of  crystallized  lead  nitrate 
(Pb(NO3)2).  The  reaction  proceeds  rather  rapidly, 
and  the  first,  very  fine,  strata  will  generally  be 
visible  in  the  course  of  one  hour. 


IN  TEST  TUBES.  127 

Lead  Chr  ornate  in  A  gar. — For  this  experiment  the 
agar  has  to  be  carefully  purified  in  the  following 
manner  :  Place  i  gm.  of  shred  agar  in  a  weighed 
200-c.c.  beaker  and  soak  it  in  three  changes  of  dis- 
tilled water,  allowing  eight  hours  or  thereabouts  for 
each  change.  After  the  last  lot  has  been  poured  off, 
the  total  weight  of  water  (a  good  deal  has  been 
imbibed  by  the  agar)  is  made  up  to  100  gm.  (i.e.,  the 
total  weight  to  101  gm.  -f  weight  of  beaker),  the 
agar  is  dispersed  on  the  boiling  water  bath  and 
o-i  gm.  of  crystallized  lead  acetate  (PbA'2  .  3H2O) 
dissolved  in  it.  The  sol  is  strained  through  a  plug 
of  glass  wool  and  filled  into  test  tubes  as  before. 
After  cooling,  a  solution  of  0-5  gm.  of  potassium 
dichromate  in  100  c.c.  of  water  is  poured  on.  The 
stratifications,  owing  to  the  low  concentrations,  are 
very  delicate,  but  exceedingly  numerous  and  regular. 
They  form  throughout  the  length  of  the  test  tube  and, 
as  the  dichromate  is  in  large  excess,  the  gel  is  coloured 
a  faint  yellow. 

As  agar  does  not  adhere  to  glass,  trouble  is  occa- 
sionally caused  by  the  aqueous  solution  creeping 
between  the  glass  and  the  gel.  This  may  be  pre- 
vented in  the  following  manner  :  a  sol  containing 
10  gm.  of  gelatin  and  3  gm.  of  potassium  dichromate 
in  100  c.c.  of  water  is  prepared.  The  test  tubes  to 
be  used  are  filled  with  the  sol,  emptied  with  constant 
turning  round  their  axis,  so  that  a  uniform  coating 
of  gelatin  is  left,  and  allowed  to  cool  with  their  open 
ends  downwards.  They  are  then  exposed  to  direct 
sunlight  or  full  daylight  for  several  hours,  during 
which  time  the  gelatin  coating  dries  and  becomes 
insoluble.  Finally  they  are  filled  with  water,  which 
is  changed  until  it  remains  quite  colourless,  emptied 
and  dried.  Agar  adheres  perfectly  to  the  tanned 
gelatin  surface  obtained  in  this  fashion. 

Many  other  reactions  may  be  studied  in  either 
gelatin  or  agar  gels,  particulars  of  which  will  be 


128  MOLAR  CONCENTRATION. 

found  in  the  literature.  The  following  points  should 
generally  be  remembered.  If  the  aqueous  solution 
is  to  diffuse  into  the  gel  at  all,  its  molecular  concen- 
tration must  be  in  excess  (generally  considerable)  of 
that  in  the  gel.  The  concentration  need  not,  how- 
ever, be  exclusively  due  to  the  reacting  solute,  but 
may  be  partly  made  up  by  an  inert  salt.  Thus  in 
the  tricalcium  phosphate  reaction,  solutions  of  cal- 
cium chloride  alone,  or  mixtures  of  calcium  and 
sodium  chloride,  give  good  results  ;  in  the  latter  the 
concentration  of  CaCl2  is  lower,  but  the  total  molar 
concentration,  CaCl2  -f  NaCl,  is  as  high,  or  higher,  as 
with  CaCl2  alone.  This  particular  expedient  always 
deserves  trial  when  solutions  at  reasonable  concen- 
trations do  not  give  good  results,  or  when  salts  of 
low  solubility  have  to  be  tried,  which  do  not  diffuse 
into  the  gel  with  sufficient  rapidity  at  the  highest 
attainable  concentrations.  Differences  in  the  quali- 
ties of  the  gelatin  used,  and  in  the  procedure  adopted 
in  preparing  the  gels,  may  affect  the  results  pro- 
foundly ;  this  is  particularly  the  case  when  the  salt 
dissolved  in  the  gelatin  is  not  neutral  in  the  con- 
centrations employed,  e.g.,  Na3PO4.  Agar  is  less 
variable  and  is  also  much  less  affected  by  many  sub- 
stances which  attack  gelatin,  such  as  acid  or  alkali 
liberated  by  hydrolysis,  so  that  it  is  to  be  preferred 
when  possible.  A  reaction  which  gives  good  results 
in  gelatin,  however,  generally  does  not  do  so  in  agar, 
and  vice  versa  ;  thus  Liesegang's  reaction  does  not 
lead  to  good  stratifications  in  agar,  while  the  lead 
iodide  reaction  does  not  produce  them  in  gelatin. 

It  is  nevertheless  sometimes  possible  to  obtain 
stratifications  with  a  combination  that  does  not 
produce  them  directly,  by  the  intermediate  formation 
of  a  reaction  product  which  appears  in  that  form. 
Two  examples  may  be  tried  as  follows  : — 

Liesegang's  Silver  Chloride  Rings. — Disperse  2  gm, 
of  gelatin  in  20  c.c.  of  water  and  add  I  c.c.  of  20  per 


SECONDARY   STRATIFICATIONS.      129 

cent,  solution  of  AgN03.  Cover  a  glass  plate,  about 
5"  X  7",  with  the  sol  and  allow  it  to  set.  Then  place 
on  it,  in  the  manner  described  for  the  silver  chromate 
experiment,  a  large  drop  of  20  per  cent,  solution  of 
NaCl.  The  latter  diffuses  into  the  gel  and  forms 
AgCl,  which  is  deposited  as  a  continuous  zone.  If, 
however,  a  few  small  grains  of  silver  chromate 
(i.e.,  the  precipitate  of  varying  composition  obtained 
by  mixing  solutions  of  dichromate  and  silver  nitrate) 
are  placed  at  points  about  10  to  15  mm.  from  the 
edge  of  the  original  drop,  rings  of  silver  chloride  are 
formed  beyond  them  as  the  NaCl  diffuses  to  that 
distance.  This  is,  of  course,  due  to  the  formation  of 
sodium  chromate  and  dichromate,  which  diffuses 
into  the  gel  containing  AgN03,  with  the  formation 
of  the  usual  rings,  which,  however,  are  transformed 
into  AgCl  when  the  solution  of  NaCl  reaches  them. 

Lead  Chromate  in  Gelatin. — Begin  the  Liesegang 
experiment  exactly  as  described.  When  a  ring  of 
silver  chromate  2  or  3  mm.  wide  has  formed,  remove 
the  drop  of  silver  nitrate  completely  with  blotting 
paper,  without  spreading  it,  and  replace  it  by  a  drop 
of  30  per  cent,  lead  nitrate  solution.  The  lead 
replaces  the  silver  in  the  chromate  formed,  while  the 
resulting  AgNO3  diffuses  ahead,  forming  fresh  rings, 
etc.  Lead  nitrate  placed  directly  on  the  dichromate 
gelatin  does  not  form  rings,  but  only  a  continuous  band. 

Reactions  in  Silicic  Acid  Gel. — Two  methods  are 
possible.  A  silicic  acid  sol  is  prepared  in  the  manner 
described  above,  and  the  one  reaction  component 
dissolved  in  it  to  the  required  concentration ;  the 
sol  is  then  filled  into  test  tubes  and  allowed  to  set. 
This  method  is  attended  with  several  difficulties. 
Some  salts,  e.g.,  iodides  or  thiocyanates,  retard  the 
setting  very  considerably.  Repeated  heating  of  the 
sol,  but  not  to  boiling,  may  reduce  the  time  required 
for  coagulation.  Other  salts,  such  as  carbonates 
and  phosphates,  promote  setting  to  such  an  extent 


130        REACTIONS  IN  SILICIC  ACID. 

that  they  frequently  cannot  be  dissolved  completely 
before  it  occurs. 

The  alternative  method  is  to  decompose  the 
sodium  silicate  with  the  acid  of  which  it  is  desired  to 
form  insoluble  salts,  and  to  use  directly  the  gel  thus 
obtained,  which,  of  course,  contains  the  sodium  salt 
of  the  acid  used.  A  10  to  15  per  cent,  solution  of 
crystallized  sodium  silicate  is  a  suitable  starting 
material  (prepared  with  boiled  distilled  water  and 
filtered,  if  necessary).  A  dilute  acid  is  then  pre- 
pared, containing  in  a  given  volume  approximately 
the  amount  necessary  to  decompose  the  sodium 
silicate,  calculated  as  Na4Si04,  contained  in  the  same 
volume  of  silicate  solution.  A  preliminary  trial  is 
then  made  by  adding  to  a  known  volume  of  the 
dilute  acid  some  methyl  orange  and  titrating  with 
the  sodium  silicate  solution  until  the  mixture  is  just 
neutral.  It  is  then  set  aside  and  allowed  to  coagu- 
late ;  if  coagulation  occurs  within  a  reasonable  time, 
say  12  hours,  the  ratio  of  acid  to  silicate  may  be 
adopted.  The  necessary  quantities  of  dilute  acid 
and  sodium  silicate  are  thoroughly  mixed  (of  course 
without  the  addition  of  any  indicator),  and  the 
mixture  poured  into  test  tubes  and  allowed  to  set, 
when  the  aqueous  solution  is  poured  on.  The 
following  give  fine  results  :  gel  obtained  by  decom- 
position with  HC1  and,  therefore,  containing  NaCl, 
on  it  25  per  cent,  solution  of  Pb(NO3)2 ;  gel  obtained 
by  decomposition  with  phosphoric  acid  and  con- 
taining sodium  phosphates,  on  it  20  to  30  per  cent, 
solutions  of  CuSO4,  BaCl2,  Sr(N03)2,  MnCl2,  etc. 
Many  other  combinations  will  readily  suggest  them- 
selves. 

Preservation  of  Specimens. — The  plates  obtained 
by  Liesegang's  method  are  allowed  to  dry  and  may 
then  be  kept  indefinitely  in  a  dry  place.  If  exposed 
to  the  atmosphere  the  silver  chromate  is,  however, 
superficially  transformed  into  sulphide,  which  shows 


PRESERVATION  OF  SPECIMENS,      131 

the  colours  of  thin  films.    This  may  be  prevented  by 
a  cover  plate  cemented  on  with  Canada  balsam. 

The  specimens  in  test  tubes  must  be  kept  from 
drying  and,  in  the  case  of  gelatin,  from  putrefaction. 
It  is  advisable  to  harden  the  latter  with  a  2  per  cent, 
solution  of  formaldehyde.  The  aqueous  solution 
left  in  the  test  tube  is  poured  off  and  replaced  by  the 
formaldehyde  solution,  which  is  allowed  to  diffuse 
into  the  gel  for  three  or  four  days  and  is  then  poured 
off.  The  tubes,  as  well  as  those  containing  agar 
specimens,  may  then  either  be  drawn  out  and  sealed, 
or  closed  with  corks  covered  with  paraffin  or  sealing 
wax.  If  the  tubes  are  sealed  off,  this  must  be  done 
very  slowly  so  as  not  to  form  a  vacuum  in  the 
upper  half  of  the  tube,  since  this  causes  the  forma- 
tion of  gas  bubbles  in  the  gel,  which  disfigure  the 
preparations. 


NAME    INDEX. 


Amberger,  39,  40 
Andersen,  88 

Bancroft,  93 
Bechhold,  69 

Chardin,  45 
Coehn,  83 
Coignet,  43 
Cotton,  88 

Estrup,  123 

Faraday,  33 
Freundlich,  93,  123 

Georgievics,  123 
Gericke,  56 

Goppelsroeder,  114,  115 
Graham,  16 
Gutbier,  33 

Hofmeister,  58,  59 
Ishizake,  93 
Jentzsch,  77,  79 

Koch,  123 
Kohlschuetter,  32 


Kruyt,  93 

Lea  (Carey),  31 
Liesegang,  50,  124,  125 
Loeb,  56 
Long,  36 

McBain,  123 
Mecklenburg,  80 
Mouton,  88 

Nelson,  43 
Nernst,  83 

Oden,  93 
Oryng,  123 

Ostwald,  Wilhelm,  100 
Ostwald,  Wolfgang,  29,  36   72 
109 

Powis,  36 

Speck,  93 
Svedberg,  35 

Tyndall,  76 
Ubbelohde,  100 
Zsigmondy,  25,  30,  33,  97 


SUBJECT-MATTER   INDEX. 

PAGE 

Adsorption    .          .          .          .          .          .          .  no,  116 

electrical      .          .          .          .          .          .          .no 

isotherm       .          .          .          .          .  116,  119 

qualitative  experiments  .          .          .       110,111 

selective       .          .          .          .          .          .          .      in 

Agar  gels 51 

sols       .........       50 

Albumin        ........  57,  97 

coagulation  at  liquid  interface  ....       58 

crystallized       .          .          .          .          .          .          .61 

dialysis  of         ......  60,  62 

dried  egg 57 

electrolyte  coagulation  of  .          .          .          .  58,  59 

heat  coagulation  of  .          .          .          .          .          -57 

purification  of  .....  60,  61 

Arsenic  sulphide  sol        .......       34 

Brownian  movement      .          .          .          .          .          .    37,  64,  79 

Cadmium  sulphide  sol    .          .          .          .          .          .  -33 

Calcium  phosphate,  strata  in  gelatin         .          .  .126 

Capillary  analysis            .          .          .          .          .          .  .113 

Cataphoresis          .          .          .          .          .          .          .  .81 

microscopic  method     .....       85 

U-tube  method  .          .          .          .          .  .81 

Collodion      ........  22,  70 

acetic  acid       ......  24,  70 

ether-alcohol  .          .          .          .          .          .  .22 

Dialysis         .........        16 

with  continuous  flow  .          .          .          .  19,  25 

of  sols  showing  osmotic  pressure  .          .          .27 

Electrolytes,  precipitation  of  albumin  sols  by    .          .  58,  59 

emulsions    ....       64 

suspensoid  sols     .          .          .89 
Emulsions     .          .          .          .          .          .          .          .          .63 

pure  oil- water         .          .          .          .          .  63,  64 

with  soap  solution  .         .          .         .          64,  65 

Emulsoid  sols        ........       41 

Ferric  hydroxide  sol       .......       85 


134  SUBJECT-MATTER  INDEX. 

PAGE 

Gelatin          .  ......       42 


brands  of  . 
gels  . 

hardness  of 
melting  point 
setting  point 
sols  . 


Gels  agar 


gelatin 
silicic  acid 
strains  in  elastic 


Gold  numbers 


•  43 

•  44 

•  45 
46>  47 
47,48 
44.  53 

5i 

44 

.       42 

.       49 
97 


sols  by  formaldehyde      ......       30 

tannin        .......       29 

various  methods  .         .         .         .         .33 

Hysteresis  (of  setting  point)    .          .          .          .          .          -47 

Lead  chromate  (strata  in  agar)         .          .          .          .  .127 

iodide  (strata  in  agar)     .          .          .          .          .  .126 

Liesegang  phenomenon            .         .         .         .         .  .124 

original  formula  for       .          .  124,  125 

preservation  of  specimens      .  .130 

Lyotropic  series     .                   51,  52 

Mastic  suspension  .......       36 

Melting  point  (of  gelatin) 45 

Optical  methods  of  examination      .....       76 

Organosols    .........       39 

Osmotic  pressure,  sols  showing         .         .         .         .          27,  62 

Palladium  sol 31 

Paraboloid  condenser     .......       79 

Parchment  paper  .         .         .         .         .         .         .16 

bags 17 

thimbles      .         .         .         .         .         .21 

tubes 19 

Phosphates  (strata  in  silicic  acid)     .         .         .         .         .130 

Polarizing  apparatus  for  examining  gels  ....       49 

Protection    .........       96 

Prussian  blue  sol 34 

Setting  point  (of  gelatin)         ......       47 

Silicic  acid  gel        ........       42 

sol .         .41 

Silver  chloride  (strata  in  gelatin)      .         .         .         .         .128 
chromate  (strata  in  gelatin)   .         ..         .         .         .124 

sols 31 

Carey  Lea's  method         .         .         .         .  31 


SUBJECT-MATTER  INDEX.  135 

PAGE 

Silver  sols,  Kohlschuetter's  method  ....       32 

by  tannin      .......       32 

with  wool-fat          ......       39 

Suspensions .........       37 

Suspensoid  sols      ........       29 

electrolyte  precipitation  of  .          .89 

mutual  precipitation  of    .          .          .          .94 

Swelling  of  gelatin          .......       43 

in  electrolyte  solutions         ...       52 

Thermostat  .........     104 

Toluene  regulator  .          .          .          .          .          .         .104 

Tyndall  cone          ........       76 

polarization  of   .         .          .          .          .  77 

Ultra-filtration 69 

Ultra-filters  for  pressure          .          .          .          .          .  69,  70 

spontaneous         .          .         .          .          .          .74 

for  vacuum  ......       72 

Ultra-condenser     ......  -77 

Ultra-microscopic  observation  with  dark-ground  condensers       79 

Vessels          .........         9 

choice  of    ........         9 

cleaning  of          .          .          .          .          .         .          .  9,  10 

Viscosity  measurements  .          .          .          .          .         .100 

Viscometers,  Ostwald's  .          .          .          .          .         .          .100 

Ubbelohde's        .          .         .         ,          .         .105 

Water 13 

distilled 13 

redistilled  .  .  .  .  .  .  .  .13 

Wool-fat 39 


THE   WHITEFRIARS   PRESS,   LTD.,   LONDON   AND   TONBRIDGE. 


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Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


3V  2     1954  LU 


REC'D 

NOV3    t956 


LD  21-100m-9,'48(B399sl6)476 


LIBRARY  USE 


JUL111957 


REC'D  LC 


